Choosing map datum

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Choosing map datum

Postby peregrinator » Fri 01 Jan, 2021 11:12 am

I'll start by quoting the eminently quotable keithy, from 9 June 2015 (Subject: Garmin Etrex 20 GPS Help):

Familiarise yourself with the different Position Formats (make sure you are using the correct format and datum corresponding with your paper maps) so that when you enter a waypoint manually, you are actually entering the intended position. Using the wrong datum, for example, will result in an incorrect location.

As it happens, I'm using that Garmin model, but my question relates to any GPS device.

I am creating waypoints from an old AGD66 1:25k paper Vicmap, to enter into my device pre-loaded with andrewp's latest Vicmap and OSM maps. (Thanks once more to the equally eminent andrewp.) I assume the latter maps are GDA94. Prior to my walk, I am converting the AGD66 coordinates to GDA94 coordinates. When I am walking, I'll be combining these converted coordinates with a recently downloaded GPX file of a route in the same locality. Therefore I will obviously have the Garmin position formats set for the entire trip as UTM and GDA.

Is the way I'm planning this correct? Another way to proceed would seem to be to reset the position format to AGD66 for one part of the walk, then change to GDA for another section. Unless I'm misunderstanding something, that way I'd avoid having to do the conversions at home before going out. Which procedure is better?

Note, I'm far from proficient with GPS theory, so a lot of the info I've read online about this issue is more or less meaningless for me.
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Re: Choosing map datum

Postby neilmny » Fri 01 Jan, 2021 12:01 pm

In the past I set my Etrex Vista Hcx to AGD66 then added the waypoints. These were converted by the GPS to GDA94 when I changed the Etrex back to GDA94. This could also be done done through Mapsource set to AGD66 while entering the points ten changing to GDA94 before uploading to the GPS which was set to GDA94.
I haven't tried this on my Etrex 30x as in batch loading them. But if you manually enter them with the GPS set to AGD66 they should be converted to GDA94 when you set the GPS to GDA94.
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Re: Choosing map datum

Postby GBW » Fri 01 Jan, 2021 12:17 pm

If I was carrying the old map and referring to it I'd want the coordinates to match the GPS, otherwise I'd convert everything to GDA94/GDA2020 and use a map that matches.
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Re: Choosing map datum

Postby johnrs » Fri 01 Jan, 2021 12:42 pm

Yes PG
As long as your device is set to the matching datum when you load the waypoints it will switch them into new formats when you change the settings.
You can do it manually using the rhyme " from old to new add 1 plus 2", ie on a six figure coordinate add one the third Easting and two the third Northing.
See here https://ozultimate.com/canyoning/map_grids.htm
And there are lots of tools online viz https://www.binaryearth.net/AusDatumTool/index.php
Best wishes
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Re: Choosing map datum

Postby commando » Fri 01 Jan, 2021 5:06 pm

WGS84 is the world standard geodetic datum
MGRS Military Grid Reference System is also an under utilised or understood reference datum also located on all Topographic Maps
The Wombat State Forest Map is better referenced in MGRS as its much easier to pinpoint a very small area to locate
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Re: Choosing map datum

Postby bernieq » Fri 01 Jan, 2021 7:39 pm

Peregrinator, ignore the co-ordinate system of the (electronic) map - it's embedded in the img file so the GPS knows what it is (you are correct, though, andrewp's IMGs are GDA94 but as you change the datum/grid on the GPS, the map changes too).

Just ensure that the map datum and grid of the paper map (from which you extract the co-ordinates) is what you have set in the GPS before you enter the co-ords.

However, there is a much easier way to load waypoints into a GPS. Eg, with a PC and Basecamp, you can simply click on each location on the map (in Basecamp or any other mapping app) to create the waypoints, export them to a gpx file on the PC and copy the file into the /gpx directory on the GPS.

Doing it this way is easer and less prone to error - and you don't have to think about the datum/grid settings as the GPS knows what the (electronic) map is and what the gpx file is and adjusts the waypoints to suit.

(edited to clarify paper vs electronic map)
Last edited by bernieq on Sat 02 Jan, 2021 12:16 pm, edited 2 times in total.

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Re: Choosing map datum

Postby keithy » Sat 02 Jan, 2021 7:45 am

It's been a spell since I've used older datum paper maps, but this is what I used to do when I didn't want to make errors in manually converting from AGD66 to GDA94:

  • On the GPS unit, set the datum matching the paper maps - so with your AGD66 maps, I would set the eTrex via > Position Format > Aus Geod '66
  • Then take your waypoints determined from your AGD66 maps and enter the waypoints into the eTrex
  • With Andrew's OSM maps installed on the Etrex, you can now use the map display screen on the eTrex to verify your waypoints against the expected position on your paper maps. This cuts down on the common waypoint entry errors especially prevalent when GPS units could not display detailed maps.
In this way you can let the GPS unit do the conversion from AGD66 to GDA94 (which also cuts down the chances for mistakes from manual conversion or transcribing the coordinates).

All your "converted coordinates" derived from the AGD66 paper maps will now display on the GPS, and if you changed the Datum or the Position Format in the eTrex settings, the coordinates will change automatically to reflect that Datum/Position Format.

So, once you're out on your walk, it doesn't matter what datum you leave the GPS in. *Unless of course you intend to mark waypoints on the paper map with coordinates from the GPS, or intend to share/relay the coordinates to a third party who might have a different paper map (and different datum).

Any new waypoints saved on the GPS (either automatically during track record or manually via "save waypoint") can be displayed under either datum, which will give you the corresponding coordinates per each datum. You can review the coordinates in either datum at any point during or after the walk, when you are reviewing the waypoints later after your walk.
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Re: Choosing map datum

Postby tom_brennan » Sat 02 Jan, 2021 5:48 pm

If you're in NSW (https://maps.ozultimate.com/) or Tas (https://maps.ozultimate.com/archive/tas/) you can zoom in, plot waypoints directly on the (latest) maps, save them as a GPX file and then copy them to your GPS. No need to worry about dealing with the changing of datums.

Unfortunately, Vic and other states seem to be struggling more with the concept of open data...
Bushwalking NSW - http://bushwalkingnsw.com
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Re: Choosing map datum

Postby peregrinator » Sat 09 Jan, 2021 4:39 pm

Many thanks for all of these helpful replies. I had no idea that a device was capable of changing datum in the way described.

Only one aspect now puzzles me. Johnrs, you mentioned AusDatumTool. I have up to now been using that. But now I don’t understand why that tool actually exists, given that one’s GPS device can convert the datum instead.
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Re: Choosing map datum

Postby Equivocator » Mon 11 Jan, 2021 8:39 pm

peregrinator wrote:But now I don’t understand why that tool actually exists, given that one’s GPS device can convert the datum instead.


It existed well before any handheld device could do it on the fly and a lot of people prefer to use the tools to which they have become accustomed to.
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