Reading a Map
Marginal Information
Printed around the margin of the map is the information needed when the map is being used. This is called marginal information.
The diagram below is an example of how marginal information may be displayed. However, this varies from map to map and between series of maps.
Map title | It may be the name of an important town or an area and indicates roughly the location of the map |
Type of map | Describes the type of map, e.g. topographic, orthophoto, geological |
Map edition and sheet number | Identifies the map edition and the map numbering system (map number) |
Grid reference block | Describes how to determine a six figure grid reference |
North points diagram | Shows, for a given year, the relationship between true, magnetic and grid north and their variations from grid north over time |
Symbols (or legend) block | Gives a legend of the symbols used on the map to represent various features, together with their meanings |
Control and production block | Gives map production details including the reliability, grid(s) shown, datums adopted |
Index to adjoining sheets | Gives the title and number of adjoining maps |
Representative fraction | Method of indicating the scale of the map |
Linear scale | Assists in the measurement of distance |
Contour interval | Vertical distance between contour lines on that particular map |
Date of field revision | When the map was ground-truthed, i.e. how up-to-date various features are |
Marginal information
Map Symbology
Each map has a key or legend.
Conventional signs
Datum
Mapping and coordinate systems are based on a datum, which is a mathematical surface that best fits the shape of the Earth. In 1966 the Australian Geodetic Datum (AGD) was defined. This datum best fitted the shape of the Earth for the Australian mainland.
In 1984 some Australian States adopted an updated version of this datum, known as AGD84. The AGD84 coordinates are based on the same datum as AGD66 and for map reading and navigation purposes can be regarded as the same.
From the year 2000, all Australian mapping authorities are using a new datum, the Geocentric Datum of Australia (GDA). This new datum was defined in 1994, and is based on a mathematical surface that best fits the shape of the earth as a whole, with its origin at the earth’s centre of mass, hence the term ‘geocentric’.
The primary reason for this change is the widespread use of satellite-based navigation systems such as the Global Positioning System (GPS), which is based on the geocentric datum known as the World Geocentric System 1984 (WGS84). For the most practical navigation purposes WGS84 and GDA coordinates may be regarded as being the same.
WGS84 is the most common default datum in GPS.
A major implication of this change is that GDA coordinates (latitude and longitudes and eastings and northings) differ from their AGD predecessors by approximately 200 metres in a north-easterly direction.
GDA Logo
Look for the GDA logo on your topographic map. If it is not present, check the datum used. Remember your GPS may show locations as 200m different if the map is not based on the GDA.
Scale and Measuring Distance
The scale of a map can be expressed as a representative fraction or as linear scale. This relationship is constant, in whatever direction the distances are measured on large scale maps such as the 1:25 000 series.
Methods of Expressing Scale
There are two main methods of expressing the scale of a map:
Words
One centimetre represents one kilometre.
Representative Fraction (RF)
This method expresses the distance on the map as a fraction of the corresponding distance on the ground. If the scale is 1:100 000, every distance on the map is 1/100,000^{th} of the distance on the ground, e.g. 1 cm on the map represents 100,000 cm or 1 km on the ground.
Common scales for Australian topographic maps are:
Scale | Ground Distance of 1 cm on the Map | |
Smaller | 1:10 000 | 100 m |
1:25 000 | 250 m | |
1:50 000 | 500 m | |
1:100 000 | 1 km | |
Larger | 1:250 000 | 2.5 km |
1:1 million | 10 km | |
1:5 million | 50 km | |
1:10 million | 100 km |
The numerator of the RF is always 1. The larger the denominator of the RF, the smaller the scale. Figure 14 is a segment of a 1:50 000 scale map of Yass. In this case, one centimetre on the map represents 50,000 centimetres or 500 metres on the ground.
The larger the scale of a map, the smaller the area that is covered and the more detailed the graphic representation of the ground. So, for example, small scale maps (such as 1:250 000) are good for long distance vehicle navigation, while large scale maps (1:25 000) are ideal for travel on foot and RFS vehicles as these show the detail required by walkers and for fighting fires.
1:50 000 Yass
1:100 000 Yass - Segment of a 1:100 000 scale map of Yass
1:250 000 Yass - Segment of a 1:250 000 scale map of Yass
Linear scale
This scale is drawn to assist in measuring distances and is worked out in kilometres and fractions of kilometres. The diagram below shows two examples of linear scale.
Linear scale
Measuring Distances on a Map
There are many ways of measuring distances on a map: using dividers, a length of string, a ruler, a scale etc. Two simple methods using a strip of paper are described below.
Measuring Straight Distances
To measure the distance in a straight line between two points on a map:
- Lay the straight edge of a piece of paper against the two points and mark the distance on the paper.
- Lay the paper along the linear scale with the right-hand mark against one of the primary divisions and the left-hand mark against the secondary divisions to the left.
- The total distance in this case is 600 metres (Figure 18).
Measuring a straight distance
Measuring Distance along a Road
It is often necessary to measure a distance that is not straight, e.g. along a road or river. To calculate the distance from A to B (Figure 19), consider the road as a number of straight or nearly straight sections:
- Lay a piece of paper along the first section and make two marks: the first at A and the second at the end of the straight section.
- Pivot the paper about the second mark until it lies along the second section.
- Mark the end of the second section and continue this method until B is reached.
- Record the total distance by road as a straight line on the piece of paper and read it off against the linear scale. The distance is about 675 metres.
Measuring a distance along a road
Map Coordinates
Map coordinates are used to identify your location or the location of a point of interest anywhere in the world. Map coordinates are usually shown or communicated in one of two ways:
- As latitude and longitude; or as
- Grid coordinates or grid references
Latitude and Longitude
These are commonly measured in degrees (°), minutes (') and seconds ("), however it is more convenient to send and receive latitude and longitude in degrees and decimals of a degree, or degrees and minutes and decimals of a minute.
For example, the latitude and longitude values can be represented as:
- Degrees, minutes & seconds e.g. 35°55’47.1″S, E148°28’42.1″,
- Decimal degrees e.g. 35.92976°S, E148.47835° , or
- Degrees & decimal minutes e.g. 35°55.786′S, E148°28.701′
Latitudes and longitudes
Latitude is the angular expression of the distance north or south from the equator (0° latitude). The South Pole is at 90°S; the North Pole is at 90°N. Longitude is the angular expression of the distance east or west from the imaginary line know as the Prime Meridian – 0° longitude on most maps. Each degree is then divided into 60 minutes; each minute is divided into 60 seconds.
Because of our location in the southern hemisphere all Australian latitude and longitude coordinates are given as south (S) and east (E).
Determining Latitude and Longitude on a topographic map
Latitude and longitude coordinates are shown in each corner of a topographic map. Black and white intervals along the edges of the map indicate the minutes of latitude and longitude.
1:25 000 Topographic Map (KATOOMBA 8930-1S)
showing the minute squares
If you are required to give a location of the fire in latitude & longitude (say when requesting aviation support), the fire location is quickly determined by identifying the degrees and minutes in the corners of the map and counting the black and white minute intervals along the edge of the map.
In this example, this fire is located in the minute square 33^{o}44’S 150^{o}29’E or more precisely 33^{o}44.3’S 150^{o}29.1’E.
UTM Grid System
The grid system shown on all Australian Topographic maps is Map Grid of Australia (MGA) and is based on the Universal Transverse Mercator Projection or UTM. The world is divided into 60 UTM grid zones (between latitude 84^{o}N and 80^{o}S). Each is given a unique grid zone designation identified by:
- A number to refer a region of longitude which is 6 degrees "wide"
- A letter to refer to a region of latitude which is 8 degrees "high"
The grid zone designation is necessary to make the coordinates unique over the entire globe.
NSW is covered by six grid zones designations – 54J, 55J, 56J, 54H, 55H & 56H (see Diagram below))
UTM Grid Zones
You can easily identify the grid zone designation for your map in the marginal information.
Grid Zone Designator on a 1:25 000 Topographic Map
Each grid zone is further divided into smaller and more accurate grids represented by eastings and northings. Typically a 1:25 000 topographic map has Universal Transverse Mercator (UTM) grid lines spaced every kilometre or 1,000 metres.
UTM grid squares on a 1:25 000 Topographic Map
A location or point of interest is given as the distance from the SW corner of a given zone. The first seven figures being the distance east of that point, the second seven being the distance north of the SW corner.
Look along the edge of the map at the labels for the vertical grid lines. The label 267000mE reads "two hundred and sixty thousand metres east” and 6264000mN reads “six million, two hundred and sixty four thousand metres north”.
Note: to ensure seven figures place a 0 in front of the 267000mE so it reads 0267000mE.
Grid References
There are four methods used to indicate positions on a map.
The method used depends on how accurate you need to be or if you require the position or point of interest to be loaded into a GPS.
Four figure grid reference
Four figure grid reference is given for the bottom left hand corner of 1 km x 1 km grid square.
This method indicates the position of one grid square only and is useful when identifying major features and localities.
To indicate a particular grid square:
- Select the easting which forms the left or western boundary of the square.
- Select the northing which forms the bottom or southern boundary of the square.
- The two figures for the easting and the two figures for the northing combined give the four figure reference required.
- This point defines a point to an accuracy of 1,000 metres x 1,000 metres and is sometimes referred to as an area reference.
Example: To give a four figure grid reference to the square that contains Point A in the diagram below
Four figure grid reference
To indicate a particular grid square:
- Select the easting which forms the left boundary of the square that contains A. In this case it is easting 31.
- Select the northing which forms the bottom boundary of the square. In this case it is northing 86.
- The four figure grid reference to the square containing A is therefore GR 3186.
Always deal with eastings first, then the northings. To help you, remember E comes before N in the alphabet
When transmitting grid references, always include the map sheet number or the title and map datum (e.g. GR 3186 Tweed Heads) to avoid confusing the number with a similar grid reference on another map or a map based on a different datum.
Grid references should always begin with the letters GR to show that they are, in fact, grid references and nothing else.
Six figure grid reference
Six figure grid reference is given for the bottom left hand corner of a 100 metres x 100 metres square.
This method is much more accurate than the four figure method and is commonly used by the NSW RFS to indicate a more accurate location of an object within a grid square.
To do this, imagine each grid square divided into 100 smaller squares (100 metres x 100 metres) and estimate which small square the particular object is in.
The diagram below shows a grid square divided into 100 smaller squares. The numbering of the lines forming the small squares indicate the number of tenths of a unit there are east of easting 31 or north of northing 86.
Point A (creek/track junction) is in the small square 31.7 east and north 86.2. In other words the easting is 31.7 and the northing is 86.2.
Deleting the decimal points, the six-figure grid reference to Point A is GR 317862. This method defines a point to an accuracy of 100 metres x 100 metres.
Six figure grid reference 317862 (A)
You can mark a piece of paper in the 100 metre increments off the scale on the map or use the 1:25 000 scale on the map card or compass to help estimate the number of tenths from a grid line to a position.
Eight figure grid reference
Eight figure grid reference is given for the bottom left hand corner of a 10 metres x 10 metres square.
This method has limited practical use and is hardly ever needed.
It is only suitable on maps with a scale of 1:50 000 or larger. It has the advantage of defining a point to an accuracy of 10 metres x 10 metres square.
UTM grid reference
To determine your exact position anywhere on the globe a UTM grid reference is required. A handheld GPS will often display your location in UTM or will require you to load a point of interest or waypoint in full UTM.
A UTM coordinate always contains the Grid Zone Designation and the exact distance in metres east and north of the SW corner of the grid zone.
If you need to determine a point of interest in full UTM to load a waypoint into your GPS:
- Determine the 6 or 8 figure grid reference and write it down
- Obtain the grid zone designator from the map of GPS (example 56H)
- Check the corner of the map to obtain the full seven digit UTM grid lines. They look like: 267000mE (MGA) and 6264000mN (MGA). Don’t forget to add a 0 in front of any single numbers to make it a seven digit number
- Insert the 6 digit grid reference into the full UTM coordinates
Note: to ensure seven figures place a 0 in front of the 267000mE so it reads 0267000mE.
Military Grid Reference System (MGRS)
The Military Grid Reference System is an extension of the UTM system.
An MGRS Grid Reference starts with the UTM grid zone designator (in this case 56H) and is followed by 100,000 metre (100 kilometre) square easting and northing identifiers. The MGRS system uses a set of alphabetic characters identifying each 100,000 metre grid squares. The 100,000 metre identifier covering the map area is found in the marginal information (in this case KH). The 100,000 metre identification is then followed by a routine six figure grid reference.
Marginal information providing MGRS grid zone identifer
Converting between Map Coordinate Systems
Converting between map coordinate systems or formats e.g. UTM grid references to latitude and longitude is achieved by:
- Plotting the location on a map in one format and manually determining the coordinates in another format
- Changing your GPS coordinate format settings and seeing the converted result
- Using a smartphone app
- Using a converter installed on your computer
- Using an online converter
Using Romer Scales
A Romer scale is a simple device used for accurately measuring the position of a point within a grid square instead of estimating the tenths.
One is supplied at the back of each NSW RFS Firefighter pocket book.
To use a Romer:
- Place the corner of the square against the required point on the map with the edges parallel to the grid lines.
- The distance east and north within the grid square can then be read off against the west and south grid lines of the square.
A different interval Romer is required for each scale of map. Romers for a variety of scales are sometimes engraved on protractors and may also be on the compass base plate of some Silva compasses.
If a Romer scale is not available, you can easily make one from a piece of paper or cardboard, marking off the appropriate subdivision of a grid square from the secondary division of the linear scale on the appropriate map. The diagram below shows the use of a Romer scale.