Friday, March 18, 2011

Bushwalking Navigation: The Importance of Using the Correct Geodetic Map Datum.

Does it matter if your GPS is set to the correct map datum? To which map datum should you set your GPS? Can you compare grid references from an old and new edition of the same map which have different map datums?  How can you convert from one map datum to another? Are the grid references in old guide books correct?
Setting your GPS and mapping software to the correct datum can make a significant difference to grid references with errors of almost 200m common. While not as important for prominent features, if you are trying to find a spur to descend from a ridge line or a specific creek junction in rugged terrain or a waterhole, accuracy can be critical.

Both your GPS and mapping software need to be set to match the map datum of your data source which can be found in the legend of the paper map you have scanned, or on the CD label of the digital map you have purchased.  Beware, the first digital edition of TopoMaps for South Australia uses the Australian Map Grid 1984 but more recent versions use GDA94.

Often when planning a walk you may have obtained the grid references (waypoints) of prominent features  from an old  bushwalking guide. However, unless you know which map datum was used in the guide, then using these may cause navigational errors, if they are inconsistent with your GPS settings. (see below for an example of differences between AGD84 and GDA94 grid references). Many of the bushwalking guides I have were written when AGD66 was being used!

HINT: if the map was published pre-1984 you can assume that the map datum is AGD66, if its publication date is between 1984 -1994, then its probably AGD84 and if its after 1994 then it's likely to be GDA94.

Adrian Heard's A Walking Guide to the Northern Flinders Ranges was published in 1990, before GDA94 and hence uses the AGD84 datum, which is the same as that used by the Third Edition (1992) 1:50K maps of the Gammon Ranges. If you are using the latest digital maps, they will be GDA94 and hence Heard's grid references will all need to be adjusted according to the formula

AGD84 to GDA94  Add 125 m to the Easting  and add 175m to the Northing

Fortunately converting from one to another is not that difficult, although not a task you would want to do when trying to find a camp site as dusk approaches. Simply open up the setting field of your GPS or mapping software and change to the correct datum, then reread the grid reference. In addition, many older maps give map specific conversions so you can convert to a more recent map datum (see below for an example)

Map Datums using for the Vulkathunha - Gammon Ranges maps

The Illinawortina (6737-3, 1:50K, 3rd Edition) map uses the Australian Geodetic Datum 1984 (AGD84) and has a sticker which says that to convert to Geocentric Datum of Australia 1994 (GDA94), add 125m to the easting and 175m to the northing (confirmed by my mapping software). The free Copley (Geoscience 1:250K) map uses map datum GDA94 (more recently adopted).

Octopus Hill, for comparison purposes, has the following UTM grid references :
0316970 6624770   visual taken from 50K map which is AGD84
0317095 6624945   when converted to GDA94
0316971 6624780   scanned 50K map and mapping software set to AGD84
0317093 6624958   scanned 50K map and mapping software set to GDA94

0316748 6624968   Copley 250K map and mapping software set to GDA94
Observations from data above:
  1. No surprises. The 250K map is not accurate enough for bushwalking navigation, with features up to 350m from their 50K map location.
  2. Using the correct geodectic datum is very important with errors of 125 m possible in eastings and 175m is northings.
  3. The last 5 digits of the GR give Thousands, Hundreds, Tens and Units of metres, so 0316748 differs from 0316970 by 222m. This means that locations on the  250K Copley map can be several hundred metres away from their true location. I have "boldened" the 6 digits usually quoted in grid references.
Related Postings

Why am I lost when I have a GPS?
Bushwalking Navigation 

  Creative Commons License This article by Bush Walker is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

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