Map Reading

There were a few things I needed to brush up on. I do not guarantee the items below are complete (I trust they're correct); they are the things that interest me. There is heaps of great information in Geoscience Australia's Map Reading Guide - How to use Topographic Maps .

Map Coordinates - Latitude and Longitude

Any point on the earth can be uniquely identified by its geographical coordinates, given as latitude and longitude. Latitude is the angular expression of the distance north or south from the equator (0º latitude). The South Pole is at 90ºS and the North Pole at 90ºN. Longitude is the angular expression of the distance east or west from the imaginary line known as the Prime Meridian - 0º longitude on all maps. Latitude and longitude are measured in degrees (º), minutes (') and seconds ("). Each degree is divided into 60 minutes and each minute is divided into 60 seconds.

Thus, Mount Tennent, near Tharwa in the ACT, could be described as being located at 35º33'5"S,149º2'36"E. Quite a mouthful.

Map Coordinates - UTM Grid

To simplify the use of maps and avoid the use of curved reference lines, a rectangular grid consisting of two sets of straight, parallel lines, equally spaced and set perpendicular to each other is superimposed on a map. Such grids are usually identified by the name of the projection used to represent the curved earth's surface on a plane. A popular projection is the Transverse Mercator projection which is the basis of the Universal Transverse Mercator grid. An excellent treatment is at Locations in Tasmania: The UTM System . To prevent area distortion, the world is divided into 60 north-south zones, each covering a strip 6º of longitude wide. The zones are numbered consecutively beginning with Zone 1 between 180º and 174ºW longitude and progressing eastwards to Zone 60 between 174º and 180ºE longitude. This gives a Zone Number.

In the middle of each of the 60 Zones is a pole-to-pole line called the Central Meridian. This Central Meridian is allocated an Easting value of 500,000 metres (this is done so that there will be no negative easting values).

The Northing value is measured, in a northerly direction, from the equator. Again, so that there will be no negative northing values, the equator is allocated a Northing value of 10,000,000 metres.

Thus, Mount Tennent, near Tharwa in the ACT, could be described as being located in Zone 55 at 685,220E (Easting), 6,063,854N (Northing).

In addition, each of the 60 zones is divided into 8º blocks of latitude. Block C covers 80ºS to 72ºS and so on northward, omitting the letters I and O. The final Block X covers 12º of latitude. This gives the Zone Letter.

The combination of Zone Number and Zone Letter provides a Grid Zone Designator.

The central slice of NSW, including all the ACT, is in Zone 55H. Now Mount Tennent, near Tharwa in the ACT, could be described as being located in Zone 55H at 85220E, 63854N. Once the zone block is specified, the leading hundred thousand can be dropped from the Easting and leading million dropped from the Northing.

Map Coordinates - MGRS Grid

The Military Grid Reference System is an extension of the UTM system. The UTM zone number and zone letter are used to identify an area 6 degrees in east-west extent and 8 degrees in north-south extent. The UTM grid zone designator is followed by 100 km square easting and northing identifiers. The system uses a set of alphabetic characters for the 100 km grid squares. Starting at the 180 degree meridian the characters A to Z (omitting I and O) are used for 18 degrees before starting over. From the equator north the characters A to V (omitting I and O) are used for 100 km squares, repeating every 2,000 km. Northing designators normally begin with 'A' at the equator for odd numbered UTM easting zones. For even numbered easting zones the northing designators are offset by five characters, starting at the equator with 'F'. South of the equator, the characters continue the pattern set north of the equator. (Complicating the system, ellipsoid junctions (spheroid junctions in the terminology of MGRS) require a shift of 10 characters in the northing 100 km grid square designators. Different geodetic datums using different reference ellipsoids use different starting row offset numbers to accomplish this.):

The UTM zone designator and the two 100 km square characters are followed by an even number of digits representing easting and northing values. If 10 digits are used, a precision of 1 metre is assumed. 2 digits imply a precision of 10km. From 2 to 10 digits the precision changes from 10km, 1km, 100m 10m, to 1m.

Thus, Mount Tennent, near Tharwa in the ACT, could be described as being located at 55H FA 8522063854. Normally, this is shortened to GR852638 (giving 100m accuracy and with the nearest same GR being 100km away - hence a 100km square two character identification would be needed and, if reporting beyond 18°, the Grid Zone Designator).

Datums

The map coordinates are anchored to the real earth via a complicated system of fixed points and a mathematical model of the earth's surface. In Australia, the anchoring system adopted in 1966 was called the Australian Geodetic Datum 1966 (AGD66). Maps based on this datum use the Australian Map Grid 1966 (AMG66 or just AMG). An updated version of the datum (AGD84) was adopted by some States in 1984, but for map reading and navigational purposes, can be regarded as being the same as AGD66.

From 2000, all Australian mapping authorities are using a new datum, the Geocentric Datum of Australia (GDA). This datum was defined in 1994. Maps based on this datum use the Map Grid of Australia 1994 (MGA94 or just MGA). The main reason for its introduction was the wide-spread use of satellite-based navigation systems such as Global Positioning System (GPS). GPS is based on the World Geocentric System 1984 (WGS84). For most practical purposes, WGS84 and GDA coordinates are the same.

GDA coordinates (both latitude and longitude, and eastings and northings) differ from AGD coordinates by approximately 200 metres in a north-easterly direction. So, to convert GDA94 to AGD66, decrease the Northing by 184 metres and decrease the Easting by 113 metres.

1:25000 Map Sheet Naming

What does the 8627 and 2S mean in a map name such as Tidbinbilla 8627-2S? A very kind and helpful lady, Pam C at Department of Lands Map & Air Photo Sales in Bathurst, provided me with an excellent explanation:

The 4-digit number is the number of the 1:100000 map containing the 1:25000 map. Each 1:100000 map covers an area of approximately 46 * 56 km or 30' of longitude * 30 of latitude'. This area is divided into 4 1:50000 maps and the map names include the 4-digit number of the 1:100000 map, plus -1, -2, -3, -4 numbered clockwise from the NE. Each of these 1:50000 maps is further divided into 2 1:25000 maps and the map names include the 4-digit number dash single digit, plus either N for north or S for south.

For example, the 1:100000 map Brindabella 8627 is divided into the 4 1:50000 maps Umburra 8627-1, Cotter 8627-2, Brindabella 8627-3 and Goodradigbee 8627-4. Umburra 8627-1 is divided into 2 1:25000 maps Taemas Bridge 8627-1N and Umburra 8627-1S. Cotter 8627-2 is divided into 2 1:25000 maps Cotter Dam 8627-2N and Tidbinbilla 8627-2S. Brindabella 8627-3 is divided into 2 1:25000 maps Brindabella 8627-3S and Bobbys Plains 8627-3N. Goodradigbee 8627-4 is divided into 2 1:25000 maps Couragago 8627-4S and Wee Jasper 8627-4N.

Geoscience Australia is responsible for 1:100000 and 1:50000 maps (plus other smaller scales) and there is an excellent online engine at

www.ga.gov.au/bin/mapserv36?map=/public/http/www/docs/oracle/agsocat/geocat.map&map_gaz_x=0&map_gaz_y=0 which allows you to search these (and other) mapping layers.

Map names are unique across a particular scale, but can be repeated in another scale. For example, there is a 1:100000 Brindabella 8627, a 1:50000 Brindabella 8627-3 and a 1:25000 Brindabella 8627-3S.

Going back up the other way, 6 * 1:100000 maps nest into a 1:250000 map. For example, the 1:250000 Canberra SI5516 map is divided into the 6 1:100000 maps Brindabella 8627, Canberra 8727, Braidwood 8827, Araluen 8826, Michelago 8726 and Tantangara 8626.

Have a look at www.ga.gov.au/nmd/mapping/number.htm for the good oil on Topographic Map Numbering.

State mapping agencies are responsible for 1:25000 maps (NSW Department of Lands - used to be Land and Property Information (LPI) - for maps covering the ACT).

SO WHAT DOES ALL THIS MEAN TO ME?

2 The most helpful form of a grid reference for map use would be a 6 figure grid reference including the map name and scale and datum. For example, Mount Tennent is at Williamsdale 8726-4N 1:25000 Second Edition GR853641 (GDA94).

3 The most helpful form of a grid reference for GPS use would be a full reference down to the metre, including the grid zone designator and datum. For example, Mount Tennent is at 55H 685333E 6064039N (GDA94).

4 Now I know what all the topographic map margin markings mean:

• the map name is WILLIAMSDALE 8726-4N
• the latitude and longitude of the map corner is 35º37' 30"S, 149º00'00"E
• the full easting and northing of the first grid lines from the corner, in black for the datum of the map, are 682000mE (MGA) and 6056000mN (MGA)
• blue tic marks with the full easting and northing of the first grid lines from the corner, in blue for the AMG datum, are 682000mE (AMG) and 6056000mN (AMG).

5 Ensure your GPS is set to the same datum as the map you are using.

6 To use a GPS UTM reference on your map, remove the first digit from the easting and the first 2 digits from the northing.

Working with ACT 1:10,000 Planning Series Maps (the ACT Grid)

The grid references relate to the ACT grid, with Mt Stromlo having co-ords of 200,000mE and 600,000mN. See http://www.actpla.act.gov.au/tools_resources/maps_land_survey/surveying_data/coordinate_system . For example, the MGA co-ordinates of Stromlo Trig (ACT E200000, N600000) are E682765, N6090037.

The map sheets referred to are the ACT 1:10,000 Planning Series and they are all available, many as 5-colour prints.

The map window extent is 8000m E-W and 6000m N-S. The sheet names are derived from the leading digits of the SW corner of the sheet. Around the map border they also have AMG ticks.

I've attached an index of the series, so your client can easily search what he needs.

There is also a conversion calculator on the bottom of the page at http://www.actpla.act.gov.au/tools_resources/maps_land_survey/surveying_data/coordinate_system .

Thanks to Alex P in ACTPLA (via Pam C in NSW Lands Dept) for the above information.

Working with ANG (Australian National Grid)

Alex P from ACTPLA is again 'da man' for providing the following information in sussing out the Mt Blundell prospect:

All is revealed. I got out an old Army 1:50,000 map and couldn't make any sense of the GR's. Then the penny dropped; the GR given is from an older Army map series, that grid being in YARDS and the Northings origin is based on 34 degrees south latitude, being 800,000 yards. In this instance, the quickest way to convert from ANG (Australian National Grid) to MGA would be by block shifts. Looking at the COTTER map, I see your site is a few hundred metres west of Blue Range Rd, just inside the ACT. For this particular area, try the following:

The "full" co-ordinates for the GR are E185900yd, N641200yd. The co-ords for the NW corner of the COTTER sheet are E176029yd, N645815yd (nearest yard, by calculation) and E659212m, N6097829m (nearest metre MGA, by calculation)
The block shift for the NW corner of the COTTER (1:50K) and COTTER DAM (1:25K) sheets is +E498,250m and +N5,507,300m (rounded to nearest 10m)
Take the yard values and convert them to metres by multiplying each by 0.9144, so 185900yd becomes 169987m and 641200yd becomes 586313m
Adding the block shift values above, your site has MGA co-ords of E 668,237m and N 6,093,613m.

Fortunately, I was able to check this by plotting both positions on the appropriate maps: they check out against surrounding detail on both maps. The best way to reference your sites (providing you have clear sky) is to hold a hand-held GPS unit over the site for about 2 minutes. That should give you co-ords based on WGS84 (which is about 1 metre different to MGA) to an accuracy of about ±5 metres.

Map Reading and Navigation Knowledge Quiz

Want to try a little quiz to check/update your knowledge? Click here (~2MB) and go to Section 5.

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This page last updated 1Jan12