Thread: Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?

Originally Posted by Swall

if you have any information about Garmin's "datum transformation" and stuff, don't hesitate to share this information here.

I can elaborate a little more based on my local situation here.

It’s likely the devices are populated with the reference frame and local datum parameters as they were fixed prior to the devices release and not subsequently updated with firmware. For example, our current datum GDA2020 was released in 2017 and my GPSMAP78sc last firmware update in May 2019 didn’t get it.

My GPSMAP66i did come with GDA2020 but it’s been poorly and incorrectly implemented. If you look at any waypoint and switch the datum between GDA94, GDA2020 & WGS84 it’s clear the shifts are wrong. They are kind of the right distance and direction between GDA94 & GDA2020 (bearing in mid no decimals are displayed so it’s not precise), however the relationships to WGS84 are wrong.

In this example and using UTM and you can see they have GDA94 still locked at the WGS84 reference frame origin where it was when originally released in 1994 as the GDA94 & WGS84 coordinates displayed are identical. But this is incorrect because GDA94 has moved with the plate and is the reason that GDA2020 was released to re-align with the framework origin. Garmin didn’t do that and the error shows us they have simply tacked on GDA2020 and calculate it as a simple offset from out of date GDA94, and as a result the GDA2020 coordinates are incorrectly offset at higher numbers.

It’s the GDA2020 coordinates that should currently at this time be the same as WGS84, and because GDA94 has shifted to the NE, the GDA94 coordinates should appear lower than WGS84.

So now both datums are in the wrong place in relation to the reference frame and any improved position accuracy in the 66 devices has already been blow away if you use it with an Australian map.

Another consideration is that the Garmin shift would not account for all the variations around the country. These transformations are calculated using a complicated mathematical formula (7 parameter similarity Helmert transformation), or commonly applied in software by a shift grid. However, the official shift grid has over 5 million points and takes over a minute to load on my laptop so that is clearly not happening in the Garmin. It’s more likely they are simply providing the same average shift amount to any point.

If you want to have a look at a shift grid there are examples of our local ones here: [Only registered and activated users can see links. ]

I would be careful trusting datums in your own area until you have tested them.