what is the orthometric datum being used by the spark fun surveyor?

GPS/GNSS Development Boards
1

When using my spark fun surveyor kit with SW Maps I see an orthometric height being shown as well as the ellipsoidal.

Can anyone tell me what datum this orthometric height is using?

Thanks.

2

Hi,

In the u-blox GNSS GNGGA NMEA message, I’m seeing 104.578m as the “alt” (Altitude above mean sea level) and 47.900 as the “sep” (Geoid separation: difference between ellipsoid and mean sea level).

In SW Map - a few seconds later - I’m seeing 152.463m as the Ellipsoidal Height and 104.556m as the Orthometric Height.

So, those numbers ~agree. Orthometric = alt. Ellipsoidal = (alt + sep). 104.556 ~= 104.578. 152.463 ~= 104.578 + 47.900.

And, to my eyes, this all agrees with https://en.wikipedia.org/wiki/Orthometric_height

We’d need to ask Tony, our expert, for a full and complete answer. But I think I’m safe to say that Orthometric Height and Altitude above Mean Sea Level are the same thing in this case.

Best wishes,

Paul

3

Regarding the datum for Mean Sea Level, this may help:

https://github.com/sparkfun/SparkFun_u- … issues/182

4

Thank you. Knowing that it is the EGM96 geoid should allow me to be able to convert to my local height datum.

5

@PaulZC that all looks correct. Just watch the signs, the orthometric height is the ellipsoidal height less the geoid height. In many areas (including all 48 US states) the geoid height is a negative number, so it is effectively addition, and I confuse this often.

H = h - N where H is the orthometric height, h is the ellipsoidal height, and N is the geoid height

Here’s a good diagram: https://www.researchgate.net/figure/Rel … _273133676

Regarding applying the geoid model of your choice, you don’t really need to know what internal geoid model the ZED-F9P in the SparkFun product is using. Just back out whatever the internal geoid model is by using the separation provided in the $GGA message and you’ll have the ellipsoidal height. Then apply your own geoid model.

The ellipsoidal height is: h = H + N

Below is a $GGA snip from a project of mine in Virginia, USA.

$GNGGA,184252.00,3811.5564944,N,07936.8396046,W,2,12,99.99,650.856,M,-33.810,M,0131*4A

H=650.856m, and N=-33.810m. That H and N are dependent on the geoid model built into the Z9P).

h=617.046 (This is the ellipsoidal height independent of the geoid model)

The geoid model I use specifies N to be -31.480 at that point, so my orthometric height is H = h - N = 617.046 - (-31.480) = 648.526m

BUT the original question asked about DATUM. That’s a different issue then geoid models and orthometric elevations.

If you’re using RTK, the horizontal and vertical datum will depend on the datum of the RTK base or the network reference station. Most network reference stations in the US are in NAD_83(2011) so the output coordinates when using those stations are in that NAD_83(2011) horizontal datum and the NAVD88 vertical datum. The ellipsoidal height and orthometric heights will be referenced to these datums.

Tony

6

Thank you Tony - much appreciated!

Paul