A map is a diagrammatic representation of an area of land or sea. It shows physical features such as constructed cities or roads, as well as natural landforms like mountains, rivers and cliffs. The purpose of a map is to depict spatial relationships between features. A map can vary from being a highly schematic piece of artwork right through to a meticulously detailed representation of geographic data depending on the purpose of the map.

Cartography is the art of creating maps, and it has evolved over many thousands of years to be as we know it today. Now maps are no longer basic cave drawings but instead produced using computers and other technologies and distributed to the masses.

People create maps for different purposes depending on their audience and what information they need to convey.

Some different map types include:

• Physical maps: Depict information on physical features like creeks, mountains and constructed features. Symbols, colours and tinting illustrate variation.
• Climate maps: General information about climate and weather in a region (e.g. rain, snow, temperature). Colour depicts variation.
• Resource maps: Show different types of natural resources or economic activity in an area. Symbols show activities; colours show differences in land elevation.
• Road maps: Show minor and major roads and other human infrastructure e.g. buildings.
• Political maps: Show human depicted political boundaries such as private properties, land tenures and National Parks.
• Topographic maps: show the shape of the landscape using contour lines. Lines far apart indicate flat ground; lines close together indicate steep ground.

Topographic or ‘topo’ maps are a specific type of map that shows the shape of the land. The map shows 3D landforms on a 2D surface and can be done via colour variation or contour lines that show elevation information too.

Topographic maps are handy tools for planning bushwalking trips and navigating through the bush. Maps enable bushwalkers to plan routes, communicate points of interest and tracks, and keep a track of where they are in relation to resources like water and shelter.

Bushwalkers love topographic maps because they:

• Simplify complex patterns.
• Facilitate visual connections between landforms.
• Enable communication with other walkers, rescue organisations, people at home, etc.
• Allow bushwalkers to extract useful info and plan a route to include campsites and water sources, avoid scrubby areas and select good walking ridges.
• Provide a universal tool of communication between bushwalkers to share trip routes, campsites, water sources, etc. with others.

All mapmakers have to overcome the fundamental mathematical problem of portraying a spherical object onto a 2D surface. Modern map-makers solve this by taking a ‘datum’, that is, a 3-D representation of the earth, and projecting it onto a 2D plane (i.e. Cartesian x-y coordinates).

This image is the typical one of the Earth centred around Europe. It is produced by converting the curved, spherical lines of longitude and latitude into a flat plane grid of perpendicular lines, but it is severely distorted at the poles.

Mercator projection of the world centred around Europe.
Purpose of image is to highlight how a 2D representation of the world is not accurate.
© Daniel R. Strebe, 15 August 2011 / Wikimedia Commons / CC-BY-SA-3.0

That’s why although Greenland appears almost same size as Australia on the world map, it’s only a third of the size:

Image of greenland superimposed on Australia.
Purpose of image is to highlight how small Greenland is relative to Australia.
© Benjamin Hell, 9 Nov 2012 / Wikimedia Commons / CC-BY-SA-3.0

No model or projection is perfect for every part of the globe, and different countries preference some projections over others to suit their needs.

In Australia, the modern MGA reference system uses the Universal Transverse Mercator projection of the Geocentric Datum of Australia 1994 (GDA94), providing a relatively universal reference system at least for the purpose of recreational bushwalking.

Becoming familiar with the elements of a map makes it easy to switch between different map types and become comfortable with reading and interpreting a large amount of data.

In general, maps contain a grid by which points can be referenced, a scale bar for relating the map to the physical world again, an indication of which direction north is and a key to determine what’s what on the map. It also contains a title and credits to the map maker, projection and geographic data used to create the map. Some maps have locators and insets.

Title
All maps contain a title in a large font that makes the map easy to identify.

Grid
In a city, it is quite simple to find a location as the streets are named and the buildings have numbers. The only thing needed is the address. However, locating places in the bush is harder because it’s not a regular daily activity. A uniform referencing system has been developed to help, and now anyone in any location around the world can accurately explain where they are to someone else.

There are the two major coordinate systems that are useful to know for recreational purposes. These are the two main sets of grid lines seen on most topographic maps: (i) the latitude and longitude system and (ii) the Universal Transverse Mercator (UTM) system.

With either system, it’s possible to communicate a location to someone else. For latitude and longitude, the system uses degrees, minutes and seconds. These are the radial positions of lines on a sphere based on a reference point (usually Greenwich, England). For UTM it’s based on a coordinate grid (Eastings and Northings, in metres), although units aren’t included when quoting a grid reference in UTM.

• Latitude and longitude lines
  Latitude and longitude lines divide the earth into a grid of circular segments: latitude lines are parallel to the equator and run horizontally around the earth. The equator is at zero degrees (0°), and lines run either side from 0° to 90° both north and south. Longitude lines are perpendicular to the latitude lines with 0° running through Greenwich, England. A location is given latitude first, followed by longitude in degrees, minutes and seconds. For instance, Sydney is 33°52.071′ S, 151°12.4392′ E. The S shows that it’s in the southern hemisphere, and E stands for how far East the point is relative to the prime meridian at Greenwich.
• The Universal Transverse Mercator (UTM)
  The Universal Transverse Mercator (UTM) system divides the earth into a perpendicular set of grid lines measuring surface distances in metres. This system reduces the complexity of transferring a location on a spherical surface to a flat surface (i.e. 2D map).
  A UTM position is given by a UTM zone followed by an Easting and Northing. Each UTM zone is 6° wide, and Greenwich England is the reference point. For example, Sydney is in the UTM Zone‎ 56H, and the centre of the city is at UTM Northing‎ 6,252,359.77, and UTM Easting‎: ‎334,895.26.

On NSW topographic maps, UTM grid lines are drawn and labelled. Latitude and longitudes are written at the edge of the map area, so it’s possible to locate grid references in terms of degrees, minutes and seconds.

Scale
The scale of a map provides a way of comparing what the map represents in terms of the actual distance on the ground.

On NSW topographic maps, the scale of the map is a ratio (e.g. 1:25,000 or 1:100,000), and the scalebar is a ruler divided into kilometres and miles. On a map scaled at 1:25,000, 4 cm on the map equals 1 km on the ground. More literally the ratio says that 1 cm equals 25,000 cm on the ground or 250 m.

For on-track day walks, bushwalkers might be able to get away with a 1:100,000 scale map. But for harder walks and walks off-track, then bushwalkers usually prefer 1:25,000. A 1: 25,000 scale is detailed enough for navigating off-track, and most trips fall over one or two maps.

Orientation
All maps are orientated to enable the user to related physical on ground features to the map. Most topographic maps used for bushwalking are orientated to true north, which means that the map aligns with the true north/south meridians that connect to the north pole. By contrast, the compass needle points to magnetic north, so when taking bearings on and off the map, the user needs to take the difference between the two norths (magnetic declination) into account.

Key/Legend
A map key or legend identifies symbols on the map and indicates what features they represent. It’s usually located in a separate box at one corner of the map and contains colours, symbols, signs or other notation to help the reader interpret the map.

On topographic maps, the key allows users to distinguish between constructed features, natural relief features, vegetation and private vs. public areas.

Credits
Map credits inform the reader on the geographical data and projections used to create the map.

Credits include:

• Cartographer’s name
• Date of map creation
• Source and date of geographic data
• Projection used

Locator/insert maps
A locator map connects the area of the map to a bigger or more recognisable (and smaller) scale. For example, Tasmania is often shown in the context of mainland Australia on a locator map. By contrast, insert maps give more detail about a particular area using a larger scale.

© Ibn Battuta 17 April 2007 / Wikimedia Commons / CC-BY-SA-3.0

There’s a lot more to topographic maps than first meets the eye. Upon inspection, a user can determine the directions which creeks flow, the depth of a water course, how likely a gully is to contain water, and so on. These can all prove useful for navigating along a track, route finding, figuring out good places to camp, collecting water and so on.

Topographic maps convey natural 3D formations in a 2D format and show how other features match to these formations. Topographic maps depict four main types of features:

• Landforms: hills, valleys, gullies, ridges…
• Water courses: rivers, swamps, coastal…
• Vegetation: national parks, farmland, plantations…
• Manmade: buildings, roads, property boundaries, political boundaries…

Landform refers to the shape of the land, and is a function of elevation and relief, that is, the representation, as depicted by the mapmaker, of the shapes of hills, valleys, streams, or terrain features on the earth’s surface. Both elevation and relief allow users to recognise landforms.

Contours and intervals
Contour lines are the most common method of showing relief and elevation on a standard topographic map, and they give a sense of slope. A contour line represents an imaginary line on the ground, above or below sea level. All points on the contour line are at the same elevation. The elevation represented by contour lines is the vertical distance above or below sea level.

The three types of contour lines used on a standard topographic map are index, intermediate, and supplementary.

1. Index. Starting at zero elevation or mean sea level, every fifth contour line is a heavier line. These are known as index contour lines. Typically, each index contour line is numbered at some point. This number is the elevation of that line.
2. Intermediate. The contour lines falling between the index contour lines are called intermediate contour lines. These lines are finer and do not have their elevations given. There are usually four intermediate contour lines between index contour lines.
3. Supplementary. These contour lines resemble dashes. They show changes in elevation of at least one-half the contour interval. Supplementary lines occur where there is very little change in elevation such as on fairly level terrain.

Shading and tinting
Shaded relief can be used to emphasise features. Relief shading indicates changes in shape by a shadow effect achieved by tone and colour that results in the darkening of one side of terrain features such as hills and ridges. The darker the shading, the steeper the slope. Shaded relief is sometimes used in conjunction with contour lines to emphasise these features.

Shaded blue areas and blue lines identify hydrography or water features. These include lakes, swamps, rivers, and creeks as well as intertidal features and reefs. Maps can also include cultural features such as shipwrecks and patrolled beaches.

Topological maps do not distinguish between saltwater or freshwater systems, although common sense can mostly be used to figure this out: coastal water bodies are likely to salt-water, inland creeks are likely to be freshwater.

Major Rivers
Major rivers are indicated as thicker blue lines and often have many smaller side creeks feeding into them. A river that has several smaller creeks feeding into it is more likely to be a permanent source of water than one without.

Perennial versus permanent water sources
Australian water sources depend on local rainfall patterns. Some rivers run all year round, while others only flow after rainfall. A perennial watercourse contains water all year round, and can be thought of as a permanent water source, whereas a non-perennial watercourse intermittently flows depending on local weather and rainfall patterns.

It’s possible to make an educated guess as to whether a watercourse is running based on local rainfall patterns, known water levels and talking to other bushwalkers that have been in that areas recently.

On NSW topographic maps, the weight of a blue line indicates whether it is perennial or not: heavy lines show perennial watercourses, fainter lines show non-perennial.

Shading and symbols show different vegetation types. This topographic map shows a small plantation area on the left between the two images.

Manmade features on a map include physical infrastructure (buildings, roads, fences, tracks) through to human-perceived boundaries (e.g. private properties vs. national parks). Here are some examples below.

Physical infrastructure
Different shapes are used to mark physical features on a map such as roads, pathways, railways lines. Check the map key for more information.

Tracks and trails
Solid orange lines indicate minor unpaved roads, and wide dashes indicate vehicular tracks. These roads are usually two-wheel drivable. Narrow orange dashes are four-wheel drive tracks. Dotted black lines are bushwalking tracks, and can vary from a well-worn concrete track right down to a faint footpad.

Some minor tracks or roads may not be accurately drawn or missing on topographic maps because these tracks overgrown or new roads have been created since the map data was collected. Checking the year of publication of the map can give an indication of how likely the tracks are to be accurate. Tracks and trails sometimes have locked gates: in general, walkers can get across locked gates but cars cannot. The exception is, of course, private property where bushwalkers must get permission to access.

Power transmission lines
Large power lines can be useful landmarks for navigation although minor power lines are likely to have changed since map data publication. Use power transmission lines to determine locations with caution.

Property boundaries
In Australia, it is illegal to trespass on private property without permission, regardless of whether a road or walking track runs through the property, or if it is the only access point to a national park.
On NSW topo maps, property boundaries are usually marked in light grey lines and are generally fields shaped into rectangles, rhombus or squares. Sometimes these properties include a number which is the designated title of the private land.
The best course of action is to plan the bushwalking route carefully and check for private property boundaries. If the track passes through a private property then a phone call ahead of time to ask for permission to enter is best. If the owners do not grant permission, do not enter.

Political boundaries
Determining where property boundaries lie enables a bushwalker to figure out what is allowed in certain areas. For instance, in NSW dogs are not allowed in national parks, but they are in state forests. Bushwalkers cannot walk on private properties without permission.

Different shades are used to mark regional boundaries (e.g. National Park boundaries, councils, state forests, etc.). Check the key to confirm regional boundaries.

Here there is a National Park boundary for the Nattai National Park, north of the grey boundary line.

Being able to interpret how topographic features fit together in the landscape gives the user a much deeper understanding of the lay of the land and how to identify where they are and how to move through it.

On topographic maps, contours represent the shape of the land. Contour lines fit together in many different ways, and they form shapes which can be recognised by the user.

Features of the landscape that are useful to know are:

• Elevation and steepness
• Hills, valleys, depression, gullies, ridges

Elevation and slope are the two elements that determine how landforms physically appear and connect.

Elevation
The ‘contour interval’ – the elevation between contours – is the vertical distance between adjacent contour lines. On 1:25,000 maps usually used by bushwalkers, contours are either 10 or 20 m apart.

For measuring between contour lines see here.

Slope (Steepness)
The rate of rise or fall of a terrain feature is known as its slope. The speed at which a bushwalking group can move is affected by the slope of the ground or terrain features. This slope can be determined from the map by studying the contour lines—the closer the contour lines, the steeper the slope; the farther apart the contour lines, the gentler the slope. Totally flat ground has no contour lines.

Four types of slopes that concern bushwalkers are gentle, steep, concave, and convex.

• Gentle: Contour lines showing a uniform, gentle slope will be evenly spaced and wide apart. Easy walking.
  

• Steep: Contour lines showing a uniform, steep slope on a map will be evenly spaced, but close together. Very challenging, or impossible walking (i.e. contour lines may be so close that they create an impassable cliff line).
  
• Concave: Contour lines showing a concave slope on a map will be closely spaced at the top of the terrain feature and widely spaced at the bottom. Bushwalkers going up the slope will find the terrain increasingly steep and challenging.
  
• Convex: Contour lines showing a convex slope on a map will be widely spaced at the top and closely spaced at the bottom. Bushwalkers going down the slope cannot observe most of the slope or the terrain at the bottom, so extra care must be taken when route finding.
  

All terrain features are derived from a complex landmass known as a ridgeline, not to be confused with a ridge.

The US Army states that “A ridgeline is a line of high ground, usually with changes in elevation along its top and low ground on all sides from which a total of 10 natural or constructed terrain features are classified”.[note]Army, U. S. “Map Reading and Land Navigation.” Washington, DC, fm: 3-25.[/note] By comparison, a ridge is a sloping line of high ground.

Major terrain features include hills, saddles, gullies, ridges, and depressions, and they each have characteristic contour lines that make it easy to pick them out in the landscape.

• Hills, peaks, knolls, mountains: A hill, peak, knoll or mountain is an area of high ground. From a hilltop, the ground slopes down in all directions. A hill is shown on a map by contour lines forming concentric circles. The inside of the smallest closed circle is the hilltop.
  • Hill = an area of high ground; generally, a smaller and rounder than a mountain, and less steep.
  • Knoll = small, rounded natural hill.
  • Mountain = a very tall hill, generally with a minimum size of 600m, but varies around the world.
  • Peak = a mountain with a pointed top.
  • Munro = a Scottish mountain taller than 3,000 feet (914 m).

• Saddle: A saddle is a dip or low point between two areas of higher ground. A saddle is not necessarily the lower ground between two hilltops; it may be simply a dip or break along a level ridge crest. When standing in a saddle, there is high ground in two opposite directions and lower ground in the other two directions. A saddle typically looks like an hourglass.

• Gully: a gully is a stretched-out groove in the land, usually formed by a watercourse, and has high ground on three sides. Depending on its size and location water sometimes flows through it, from high to low. Contour lines forming a gully are either U-shaped or V-shaped. To determine the direction water is flowing, look at the contour lines. The closed end of the contour line (U or V) always points upstream or toward high ground. A valley is a large gully, often very flat, wide and open with a large watercourse running through it.

• Ridge: a ridge is a sloping line of high ground. When standing on the centerline of a ridge, there is usually low ground in three directions and high ground in one direction with varying degrees of slope. When crossing a ridge at right angles, there is a steep climb to the crest and then a steep descent to the base. When moving along the path of the ridge, depending on the geographic location, there may be either an almost unnoticeable slope or a very visible incline. Contour lines forming a ridge tend to be U-shaped or V-shaped. The closed end of the contour line points away from high ground.

On a map, a ridge is depicted as two contour lines (often of the same contour) running side by side at the same elevation for some distance. When the lines diverge, the ridge is either flattening out to a high plateau or continues to rise with additional contour lines. When the lines converge, the ridge is falling in elevation, creating a spur.

Closed contour loops represent hills or bumps along the ridgeline.

• Spur: A spur is a short, continuous sloping line of higher ground, normally jutting out from the side of a ridge. A spur is often formed by two roughly parallel streams cutting draws down the side of a ridge. The ground will slope down in three directions and up in one. Contour lines on a map depict a spur with the U or V pointing away from high ground.

• Depression: A depression is a low point in the ground or a sinkhole. It could be described as an area of low ground surrounded by higher ground in all directions, or simply a hole in the ground. Usually, only depressions that are equal to or greater than the contour interval will be shown. On maps, depressions are represented by closed contour lines that have tick marks pointing toward low ground.

• Cliff: A cliff is a vertical or near vertical feature; it is an abrupt change of the land. When a slope is so steep that the contour lines converge into one “carrying” contour of contours, this last contour line sometimes has tick marks pointing toward low ground (image below). Cliffs are also shown by contour lines very close together and, in some instances, touching each other.
  
  Topographic maps cannot always be used to identify cliffs, however, particularly on those with 20m contour intervals, and hence some steep areas require careful negotiation.

The real art of map reading comes with interpreting how individual landscape features fit together in the terrain: saddles connect ridges to knolls to cliffs; gullies form into rivers and valleys. Interpreting how contour lines fit together helps understand the lay of the land and be able to navigate through it.

The big picture

1 – hill, 2 – valley, 3 – ridge, 4 – saddle, 5 – depression, 6 – gully, 7 – spur, 8 – cliff, 9 – cut, 10 – fill

This image describes a landscape by contours. In words:
Running east to west across the complex landmass is a ridgeline. A ridgeline is a line of high ground, usually with changes in elevation along its top and low ground on all sides. The changes in elevation are the three hilltops and two saddles along the ridgeline. From the top of each hill, there is lower ground in all directions. The saddles have lower ground in two directions and high ground in the opposite two directions. The contour lines of each saddle form half an hourglass shape. Because of the difference in size of the higher ground on the two opposite sides of a saddle, a full hourglass shape of a saddle may not be apparent.

There are four prominent ridges. A ridge is on each end of the ridgeline, and two ridges extend south from the ridgeline. All of the ridges have lower ground in three directions and higher ground in one direction. The closed ends of the U’s formed by the contour lines point away from higher ground.

To the south lies a valley; the valley slopes downward from east to west. Note that the U of the contour line points to the east, indicating higher ground in that direction and lower ground to the west. Another look at the valley shows high ground to the north and south of the valley.

Just east of the valley is a depression. Looking from the bottom of the depression, there is higher ground in all directions.

Several spurs extend south from the ridgeline. They, like ridges, have lower ground in three directions and higher ground in one direction. Their contour line U’s point away from higher ground. Between the ridges and spurs are draws. They, like valleys, have higher ground in three directions and lower ground in one direction. Their contour line U’s and V’s point toward the higher ground.

Two contour lines on the north side of the centre hill are touching or almost touching. They have ticks indicating a vertical or nearly vertical slope or a cliff.

The road cutting through the eastern ridge depicts cuts and fills. The breaks in the contour lines indicate cuts, and the ticks pointing away from the road bed on each side of the road show fills.

Common mistakes
Here are some tips and tricks to identify between standard features.

• Spur vs. gully: Contour lines on a map depict a spur with the U or V pointing away from the high ground; for a gully, the closed end of the contour line (U or V) always points upstream or toward high ground.

Spur

Gully

• Knoll vs. depression: for knolls, contour lines form concentric circles, and there is lower ground all around, whereas depressions have closed contour lines with tick marks pointing toward the low ground.

Knoll

Depression

• Saddle vs. ridge: When standing in a saddle, there is high ground in two opposite directions and lower ground in the other two directions. When standing on the centerline of a ridge, there is usually low ground in three directions and high ground in one direction with varying degrees of slope. Be careful not to confuse ‘ridge’ with ‘ridgeline’ here: a ridgeline is a line of high ground, which can rise and fall through saddle features.

Practicing
Map reading takes practice. One of the easiest ways to do this is to become aware of the shape of the surrounding land at all times, even when driving and walking through an urban area. Most navigation and map reading is about matching up the form of the land with that on the map. Practice recognising and naming key features (knoll, hill, spur, ridge, cliff, valley, etc.). Take maps on all bushwalks and follow the route on the map, even if it’s well signposted. Look at the map regularly and match it with the surrounding landscape.

Some bushwalkers enjoy taking part in Rogaine competitions to improve their navigation. The Bushwalkers Wilderness Rescue Squad also runs an annual navigation competition in NSW.

There are subtleties to map reading that take time to develop. Some common things to watch out for include:

• Implied knoll: A bump or small hill that is too small to generate it’s own closed loop contour. Occurs in places where two ridge lines diverge and converge or on the top of a hill where the contour lines are furthest apart.

• Implied saddle: The opposite of an implied knoll. An implied saddle is a saddle that is not formed enough to have two parallel contours cross the ridgeline and connect.

• Minor gully: A dent in the side of a slope or ridge often too high for a proper water course to form.

• Minor spur or outcrop: a bump on the side of a ridge or slope that’s too localised or flat to form a proper spur.

Topographic maps enable the user to take measurements of distance and elevation. The accuracy of these measurements can be in the order of metres depending on the scale of the map.

The following measurements can be taken from a standard topographic map:

• Distance
• Bearing
• Elevation change

Calculating the distance between two points on a map is done by measuring the distance with a ruler in centimetres, then converting it to metres or kilometres.

For a straight-line distance between two objects, use a ruler to measure the distance between the points. If the distance is curved, use string along the curve, then lay it on a ruler to measure the length of the string.

Find the scale bar on the map. It might be written as a fraction (1/25,000 or 1:25,000) or a bar scale. Both provide a conversion factor that must be applied to calculate the real distance. A 1:25,000 scale map says that for every 1 cm on the map it represents 25,000 cm on the ground. So if the distance measured on the map is 25 cm, the distance on the ground is 25×25,000=625,000cm or 6.25km.

The precise formula is:
(Map distance (in centimetres) x denominator of scale)/100,000 = Real distance (in kilometres)

Some compasses include a converted scale on one side of them where the measurement for 1:25,000 maps or 1:50,000 maps is in kilometres already. Having a converted scale on hand can be useful for getting rough estimates of distance quickly. Alternatively, on 1:25,000 maps, each grid represents a km, so it’s possible to eyeball the map to get a rough distance estimate without using a ruler. Using the compass or eyeballing the map is most practical out on a bushwalk.

A bearing is a number in degrees that represents the direction of one point relative to another. It’s helpful for navigating between two points or triangulating a location. On topographic maps, the bearing is relative to grid north.

For calculating the bearing of point B relative to point A, follow these steps (note that the map does not need to be oriented):

1. Draw a line on the map between points A and B.
2. Line up compass with the directional arrow pointing from A towards B. Make sure that the direction is correct, otherwise the bearing will be 180° out.
3. Rotate the movable dial so that large red arrow on the dial is parallel to the North/South grid lines on the map, and the arrow is facing north. Ignore the magnetic needle in this step. Make sure that the dial is aligned with grid north, not grid south (easy to do if the map is upside down), and that the directional arrow hasn’t slipped out of place during this step.
4. Read the dial to give a number between 0° and 360°, clockwise from north.

Note: The bearing taken from a map is the grid angle, but bushwalkers must use the magnetic angle when navigating in the field. Hence a grid bearing must be converted to a magnetic bearing before using it to navigate.

Changes in elevation can be calculated using contour lines. Here’s an example below.

The lower index contour line is numbered 200, which means any point on that line is at an elevation of 200 meters above mean sea level. The upper index contour line is numbered 300, or 300 meters. A bushwalker traveling from X(a) to X(b) will go up in elevation. The exact amount of change can also be calculated: Point X(a) is located on the second intermediate contour line above the 200-meter index contour line. The contour interval is 20 meters (there are four intermediate contour lines of 20 m between the 200 m and 300 m contour line) and hence the bushwalker will go through an increase in elevation of 40 m (240 m to 280 m).

To determine the elevation to a hilltop, point (c), add one-half the contour interval to the height of the last contour line. In this example, the last contour line before the hilltop is an index contour line numbered 300. Add one-half the contour interval, 10 meters, to the index contour line. The elevation of the hill would be 310 meters.

A grid reference indicates a location on a map in terms of numbered vertical and horizontal grid lines. Grid references are useful for sharing routes, and quickly communicating to emergency services. Bushwalkers often share grid references of campsites or camp caves, obstacles or challenges on a walk, where to get through a cliff line, where to cross a river or what route to take. Orienteering competitions also provide information on their flag locations using grid coordinates.

To share a grid reference, the name of the map must be given, plus the X and Y coordinates. Users must also be clear on the reference system used.

When sharing a grid reference, bushwalkers must be clear on the reference system being used, else the points will not necessarily match up. Maps that show the same terrain can be made using different datums and projections, meaning that the grids do not necessarily line up. 

For example, compare the images of the Megalong Valley below, and notice that the location of all features relative to the grid lines is different. This is because the first map below uses an older datum than the one below that. 

AMG

MGA / GDA94

Any given reference system is based on a datum, a 3D representation of the earth, and datums differ in how they represent land and where the location of the central frame is. Two points can be out by as little as a few hundred metres, or as much as several kilometres if there is confusion over the reference system in use.

Maps usually have an information section that identifies the map datum and projection, along with the publisher and copyright information. When communicating a grid coordinate, state the coordinate system first, then the grid coordinates.

In NSW, the 1:25000 topographic maps use either the AMG or the MGA coordinate systems.

• AMG: The AMG is the Australian Map Grid 1966/1984 system and was used on the old series maps, mostly distributed circa the 1980s. The AMG reference system uses the Universal Transverse Mercator projection of the Australian Geodetic Datum 1966.
• MGA: The MGA is the Map Grid of Australia 1994, and is used in all the new series maps. As a general rule, all the new series maps have an aerial photo image on the reverse. The MGA reference system uses the Universal Transverse Mercator projection of the Geocentric Datum of Australia 1994 (GDA94).

The AMG system was used until roughly the mid-90s when it was replaced with the MGA system, which is more compatible worldwide as the GDA 94 datum is almost identical to the WGS84 datum used in GPS (Global Positioning Systems). The result of the change from the AMG system to the MGA system is a shift of approximately 200 metres in a northeasterly direction.

While being off by 200 metres doesn’t sound like much, in the bush this can be disastrous for an emergency operation, or cost a bushwalking party serious time delay. That’s why it’s important to communicate which reference system is being used and to know how to convert between them. In an emergency, it may be possible to get enough mobile phone coverage to contact the emergency services and communicate the location of rescue.

The shift between AGM and MGA is approximately 200m in a northeasterly direction. The images below give an example of the shift relative to the UTM Northing and Easting grid lines.

AMG

GDA

To convert between grid points, users add 100 m to the Eastings, and 200 m to the Northings. As a practical example, for a 6-digit grid reference, add 1 to the Eastings (the first three digits), and 2 to the Northings (the second three digits). So the junction of the two roads Hampton-AMG436645 becomes Hampton-MGA437647. The examples below have more information on reading grid references.

Since all objects on a map remain the same relative to each other between the two systems, bearings do not change.

Maps have numbered vertical and horizontal grid lines that enable the reader to identify and communicate a particular location on the map. The vertical lines are aligned with grid north, and the horizontal ones are exactly perpendicular to the vertical lines.

The Universal Transverse Mercator (UTM) system, which describes a point in terms of metres, is most commonly used by modern topographic maps. And this is the system described here.

In the UTM system, the vertical lines are called Eastings (i.e. they help locate how far east the point is), and the horizontal lines are called Northings (i.e. they establish how far north the point is). Most maps also include the Longitude and Latitude of the map at the corners (i.e. a reading in degrees, minutes and seconds).

A UTM grid reference tells the reader how East and how far North to go on the map. It consists of two parts – the Easting digits, and the Northing digits – and always has an even number of digits. Determine the number of Eastings and Northings digits by dividing the whole grid coordinate by 2. The more digits, the more accurate the grid reference.

When reading a grid reference, Eastings always come first, followed by the Northings. A simple way to remember this is the saying ‘cross the creek before going up the tree’, that is, go horizontal first, then vertical.

Example 1: Describing the location of the hill labelled 619. The 619 refers to the height of the hill in metres. Often, bushwalkers describe a hill by its height, unless another name is written on the map (typical for popular and distinctive hills). In this example, since the hill has no name it can be called Hill 619.

On the 1:25,000 map above, each square is 1km wide and high. The Northing and Easting lines are all numbered. Ignoring the small numbers either side of the black numbers (explained later), notice that the lines increase by one unit along the bottom (68, 69, 70, 71…) and up the left-hand side of the map (05, 06, 07, 08…).

A four digit grid reference (2 digits for the Easting, and 2 for the Northing) is a crude measure of the location with an accuracy of one square kilometre. It describes the intersection of two lines. The closest intersection to Hill 619 is the intersections of the Easting line 69 with the Northing line 06. Hence Hill 619 has a four digit coordinate of 6906 plotted as a white circle below.

However, since the margin of error in a four digit this grid coordinate is 1km square (outlined in orange), this reference could describe any of the features from the hill in question, to a gully or a creek junction. To more accurately describe Hill 619, it’s best to use a six-digit grid reference.

Draw perpendicular lines from Hill 619 to intersect with the Easting and Northing lines. Then measure how far along the grid the lines are. For the Easting, the line crosses 6/10th of the way along the grid. Hence the Easting coordinate is 686. For the Northing, the line intersects 1/10th of the way up. Hence the Northing coordinate is 061. The Hill has a six-digit grid reference of 686061. Since the map is an old map (‘1:25,000 Colo Heights’) and uses the AMG reference system, the six-digit grid reference is ‘Colo Heights AMG-686061’.

A six figure coordinate has six figure grid reference an accuracy of 100 m2, indicated in the orange square box. This area of error is much smaller than the search area provided by a four-digit grid reference.

Example 2: Bonnum Pic.

Care must be taken to quote all the zeros in the right place. In this example, the Northings have a zero at the front. The 6 figure grid reference is therefore 477060. Since the map is a new edition map and uses the MGA reference system, the six-digit grid reference is ‘Hilltop MGA-477060’.

Example 3: Hill 2122. The hill lies on a grid line meaning that for a six-digit grid reference, the Easting has to include a zero at the end. Hence the six-digit grid reference of Hill 2122 is 010977. Since the map is an old map (‘1:31680 Caoura’) and uses the AMG reference system, the six-digit grid reference is ‘Caoura AMG-010977’.

Example 4: Hill 731.

An 8-digit grid reference is even more accurate than a 6-digit one and has an accuracy of 10 m2. Supplying accuracy to the nearest 10 metres involves splitting up the grids into 100-tick increments.

Hill 731 has an eight-digit grid reference of 49029349. Since the map uses the MGA reference system, the eight-digit grid reference is ‘Hilltop MGA-49029349’.

UTM zones: Identifying the region
UTM divides the earth into 60 zones each with 6 degrees of longitude, and 20 designators each with 8 degrees longitude.

© Jan Krymmel, 26 Jan 2007 / Wikimedia Commons / CC-BY-SA-3.0

Australia falls between zones 50-56, and Sydney is in 56H.

A 6-digit grid coordinate system works well among bushwalkers to communicate points of interest and routes, and Bushwalkers use a shortcut to identify the region in question. They refer to the name of the map in question, a practical and quick solution for sharing information.

However, on a global scale, there would be many identically-numbered grid locations unless the specific UTM zone is also reported, along with a context for the Easting and Northing lines.

In the corner of all maps, the Eastings and Northings are given additional small numbers before the main ones that identify the lines.

While the small numbers aren’t usually quoted by bushwalkers for six- or eight-digit grid references, they will appear on a GPS reading. GPS Eastings and Northings always include an accuracy down to the nearest metre. Hence for every square kilometre, the accuracy has to be to 3 digits for Eastings and Northings, something that is hard to do when reading off a map, but if given these grid coordinates, it’s possible to plot accurately.

Hence, the full coordinates of Hill 731 are Zone 56H, 0269210mE, 6316890mN. This is the same eight-digit grid calculated in example 4 above in orange.

Plotting a grid reference on a map is the reverse process of reading it on the map.

Example 1: Plotting the grid reference ‘Caoura AMG-041948’, the junction of a creek with a major river.

The reference 041948 is a six-digit grid reference with a 041 Easting and 948 Northing. Find the 04 Easting line and follow it 1/10th of the way further east. Then find the 94 Northing line, and follow it 8/10th of the way further north. The grid point is the junction of Paradise Creek with the ShoalHaven.

Example 2: Plotting the grid reference for Mt Wangandarry ‘Hilltop MGA-48259765’, a trig point.

The reference 48259765 is an eight-digit grid reference with a 4825 Easting and 9765 Northing. Find the 48 Easting line and follow it 25/100th of the way further east. Then find the 97 Northing line, and follow it 65/100th of the way further north. The grid point is the top of a knoll on a large flat hill top.

Tools
Coordinate plotting tools can aid plotting grid points on maps. Grid tools enable the user to keep an exact right-angle position as they find the coordinates they’re looking for, and the user does not need to draw lines on the maps.

A UTM plotting grid is one option. Alternatively, a corner style tool or simple ruler. Ensure that any map plotting tool has the right scale (note that American sellers stock 1:24,000 tools, not to be confused with the more common 1:25,000 series used in Australia).

GPS units are an excellent tool for identifying and plotting locations along a bushwalk. GPS units can also be loaded with a basic topographic map and route plan at home, and users can follow the route in the field. Alternatively, users can identify specific locations along the walk and translate them back onto a topographic map to double check their location and accuracy of navigation.

GPS units act as an additional navigation tool to traditional map and compass. They typically give accuracy up to 20 m but rely on good satellite coverage. GPS units fail in regions where the sky is partially or wholly obstructed (caves, cliff lines, canyons).

Before use set the GPS unit to the correct Geographic Coordinate System, based on the type of maps being used. The most common system that bushwalkers use is the UTM/UPS system (not latitude/longitude). Then select the appropriate datum: for new series maps use WGS 84 (or GDA 94 as they are essentially the same); for old series maps use AGD66. If unsure, read the fine print in the publication details of the map. All datum and reference modifications on a GPS can be made in the ‘settings’ section.

Reading a grid reference on a GPS

A GPS grid reference looks something like this:

It includes information on the Geographic Coordinate System used (UTM), the location zone (56H), the Eastings (0706832) and the Northings (4344683). Eastings are always given first.

The ‘706’ part of the Easting information refers to a major Easting line, and the ‘832’ gives the location down to the nearest metre. The ‘4344’ part of the Northing information refers to a major Northing line, and the 683 gives the location down to the nearest metre in the grid.

Translating a GPS reading onto a map

Example: Finding the GPS coordinate Zone 56H, 0304920mE, 0695440mN.

The map below is in Zone 56H. Major Easting lines are 04, 05, 06, etc. and all have a ‘3’ at the start. Major Northing lines have 95, 96, 97, etc. Eastings have a ‘3’ prefix shown on the map, which is ‘03’ in the GPS coordinates. Similarly, the Northing lines have a ‘6’ prefix shown on the map, which is ‘06’ in GPS coordinates.
Hence, the 8-digit grid reference is 04929544. Follow the Eastings along to the 04 line, and then 92/100th further east. Follow the Northings to the 95 line and 44/100th further north.
The location is Mount Woolnough.