Base Camp & GDA94

I wish it were that simple. I'll add a case in BaseCamp to have this datum added.

I suspect that the datum GDA is actually GDA94.

But to be honest, I doubt that you will see a difference between GDA94 and WGS84 for the digits displayed in either BaseCamp or your unit.

The GRS80 and WGS84 spheroids are nearly the same (differences will increase as you approach the poles). If GDA was treated similarly to NAD83, the correction parameters for the reference frame are zero (i.e. the origin and axes of the two reference frames are assumed to be coincident by Garmin). In truth, they may differ, but they are probably within the accuracy of GPS and the error for the method of transformation. The net result is that WGS84, NAD83 and GDA94 will almost always display the same lat/lon - at most differing by 1 in the last decimal.

I'm not familiar with your device, but if you can make a waypoint you can test it by doing so and changing the position format of your device or similarly in BaseCamp.

Technically you are correct regarding the differences between the datums. Perhaps my point / query is philosophical in nature, why provide a datum in one bunch of software but not across the platform given it is core to the process and calculations.

On a day to day level I am working with various maps in AGD66 and AGD84 and GDA94 and need to stick with a process of checking the map and checking the GPS and checking Base Camp and keep an eye out for anomalies. So it would be better to have Base Camp using the same data refs as the GPS to make this process more sure and seamless. When the datum is a different ref (WSG84 for GDA94) on each device as opposed to the same ref there will come a day when I stuff it up. And that will be on a day when I really want it to be right but will only figure out the error when things are not going right.

Technically you are correct regarding the differences between the datums. Perhaps my point / query is philosophical in nature, why provide a datum in one bunch of software but not across the platform given it is core to the process and calculations.

Without worrying (or even thinking) about all the technical details between different datums, those were my exact initial thoughts.
And "in general", this reminded me of a seemingly "lack of standardization" & communication amongst (between) so many related GARMIN GPS products/offerings.

I checked a manual for an older device which lists the names of the datums.

GDA is in fact the Geocentric Datum of Australia (GDA94 is the first and only geocentric datum for Australia at this date).

I would have been surprised that a datum that old would not be present. I am not sure when or why the name change occured (I would expect GDA94 is found on maps and therefore the proper name, but perhaps the earliest printings used GDA).

The OP should be concerned about at least some of the technical details if he is worried about "stuffing it up". Using WGS84 for AGD66 would be a clear mistake, but WGS84 for GDA94 probably a meter at most. Compare this to pulling a coordinate off a paper map. On a typical 1:25000 scale map there is 2.5m error per 0.1mm error in measurement, 1m on a 1:10000 scale.

I'll also point out that there are different methods for converting between datums and all have some error.

As far as collecting data in the field (i.e. marking points, tracks) you should have no concerns about "stuffing it up", they are always stored in the WGS84 datum, regardless of what position format is in effect.