A question of which datum?
Uploaded 6/12/00
The new handheld GPS (Garmin Etrex summit), supports a whole long scrollable list of "datums".
I've just bought a couple new marine charts - and they are in GDA94. The maps
say for GPS use wgs 84 and that I'll be up to 200 meters out. The
scrollable list of datums includes a GDA with no numbers (like 94 or whatever),
and another GDA 49.
Shouldn't a new GPS support gda 94?....should I be downloading it from Garmin
via their website as a softwear upgrade?
The salesman told me not to upgrade the software version 2.04 yet as he'd seen
one where someone had loaded the new upgrade 2.06?, and there was a bug that
caused tracklogs to be lost or something.
I'm confused...should I be aiming for better accuracy that wgs 84?..
Theres a zillion datums on the scrollable list - but which one do I choose if
GDA94 isn't among them?
Seems kinda daft having a brand new chart and brand new GPS and not being able
to do better than within 200 meters (considering selective availability is
switched off).
Am I missing something obvious?
Thanks & cheers!
ANSWER:
First issue mapping datum, most currently available Australian mapping is related to either AGD66 or AGD 84 (which usually appears as Aust 66 or Aust 84 on GPS units). Some very recent maps may be published in GDA94. For all recreational uses AGD66 and AGD84 can be considered the same. GDA94 is the Australian equivalent of WGS84 and can be considered the same for recreational users. (not for surveyors though! eh Kerry). AGD66 (or AGD84) is significantly different to GDA94 (= WGS84) and the differences are in the region of 200m to approx 215m here in Qld. Not sure of the magnitude in WA but some where in the region of 200+m.
I am also a little confused about the “note” on the maps as GDA94 is the same as WGS84, so either the map is in AGD84 (or AGD66) or the note is perhaps incorrectly applied to the map or chart.
Not sure about your “GDA” or “GDA49” datum in the unit, simple solution is to set your GPS datum to WGS84 (ie = GDA94)for charts in GDA94 or set to Aust84 for charts on AGD84 datum.
You can certainly get better than 200m positional accuracy with your GPS in the current environment with the yanks turning off Selective Availability (SA). Most mapping (maps and charts) is compiled from a variety of source data, some recent some very old. We now face the very real situation of recreational GPS units being more accurate than the source data the maps were compiled from. We could only dream of 1-5 or even 10m accuracy only a decade or so ago as surveyors and that was with some very expensive gear and very time consuming observations. Now a $300 electronic gizmo can do that in a few minutes.
Kerry just happened to bench test my GPS/Plotter unit over the transition from SA to no SA over the 1st May. Prior to SA being turned off 66% of points were within 100m of true position ( the other 34% well - up to 250m +) after SA was turned off 99% of points were within 15m of true position. So there is a significant change in accuracy with SA off!!
Response supplied by Moby
Moby’s outlined the pros and cons of the issues and differences between AGD66/84 and WGS84/GDA94.
If you read that warning again it should outline that the difference if a GDA datum is not used with a GDA chart/map then you will be about 200 metres out? Similar if a AGD datum is not used with a AGD chart e.g. with a AGD chart and GPS datum GDA94/WGS84 then about 200 metres, GDA chart (as you have) and GPS datum AGD66/84 then also about 200 metres but may not be the same direction?
Some of the newer Garmin’s have the Geocentric Datum of Australia in the datum list as “GDA” (GDA94), which for all intent and purposes is the same as WGS84. Easy to check that one by fixing a position in WGS84, then checking the same point with the datum set as GDA. Allowing for typical slight wandering (depends how quick you are at changing datum’s) the positions will/should basically be the same.
As manufacturers implement GDA we will probably see variations (as we do know) in how it’s shown in the datum list. There are some technical differences between GDA by itself and GDA94 but will stay away from that here.
Can you re-check that “GDA49” as if it is GDA49 then Garmin may have a problem? Check that it’s not “Geod Datum ‘49” (stands for Geodetic Datum 1949 New Zealand) or something similar as NZ’s equivalent to GDA94 is called NZGD2000 (New Zealand Geodetic Datum 2000). There are major differences between Geoid Datum'49 and NZGD2000.
The latest upgrade for the Summit is 2.07 (22/11/00) which, might have that route/track problem fixed (may have originated first in version 2.05?). Mention is also made in 2.07 about correcting New Zealand position format selection and some route issues.
As I don’t have a summit am interested in exactly what those datum’s you mention as listed are.
You set your brand new GPS to the same datum as your brand new chart and you can’t go wrong.
Cheers, Kerry.