Datum of a GPS Receiver

Page last updated: 14th February 2007

When using a GPS receiver for scientific purposes, it is important the user knows what Datum (and units) their GPS outputs - so coordinates can be understood and used by others.

Background:
The mathematical model that the GPS system uses is known as WGS84 ( World Geodetic System 84 ), this is used inside every GPS to determine location. However this model can vary by up to +/- 100 metres to Mean Sea level. People using early GPS receivers were often dismayed that their GPS said they were at -50 metres, when indeed their boat was still safely floating on the sea surface. So GPS manufacturers began adding the ability to give the height to MSL (Mean Sea Level) which also aligned better with existing paper maps on land.

These days, most modern GPS receivers output the latitude and longitude to WGS84 Datum, and the height to MSL (Mean Sea Level) Datum.

To see how much MSL (Mean Sea Level) and WGS84 vertical datum differs with location, Click Here to bring up a separate page, showing a color coded map of the world.

However, it is not always possible to find (in the GPS manual) what the vertical datum actually is. This set of notes will hopefully allow someone to find out - whether their GPS vertical Datum is WGS84 OR MSL.

The difference between the WGS84 ellipsoid and MSL datum, depends on where you are. Using GPS horizontal location, one of the following calculators can be used to find "geoidal separation" (vertical distance between WGS84 and MSL datum).

Let's assume a user is at Eureka (north of San Francisco) where a GPS gives horizontal position as 40.77694N , 124.18878W. Using the following calculator, we enter these coordinates:

Geoid Calculator (CONUS only)

Geoid Calculator (World-Wide)
and are told the Geoidal Separation = -30.644 metres. 

Using the expression:

Height (WGS84) = Height (MSL) + Geoidal Separation 
Height (WGS84) = Height (MSL) +  (-30.644) metres      [paying attention to sign]
We can see (for this location) the WGS84 value is going to be 30.644 metres "lower" than the Mean Sea Level height.

Using either this Interactive Google Map (click TOPO button) or using Google Earth, we find the height is reported as 9 metres (MSL). The GPS gives a height of 12 metres, so because there is only 3 metres difference between the map and GPS (rather than 30 metres) the GPS is clearly giving its height to MSL datum. Now to calculate the height in WGS 84 datum for this location:
Height (WGS84) = Height (MSL) + Geoidal Separation
Height (WGS84) =   12m  (MSL) + (-30.644)     [paying attention to sign] 
Height (WGS84) =  -18.6 metres 
The above case assumes the map we use as reference, has very accurate height. This may not always be the case, and indeed one could come to the WRONG conclusion about what the GPS height datum is, using electronic or paper maps.

In the USA, a better way is to find nearest Geodetic Benchmark to compare GPS height against.

Perhaps the best way way to find the vertical datum of a GPS - is to compare it against a known (vertical datum) system.

If you know someone with a KIWI OSD system, this can be used as a reference - because the datum issue is already known:
The height data in KIWI OSD, is presented in this form:

1  11  0.9  554.9  M  44.4

The fourth number is the height to MSL datum, the number after the M is "Geoidal Separation".

The M, indicates the units of height are in Metres (not feet).

Height (WGS84) = Height (MSL) + Geoidal Separation
Height (WGS84) = 554.9m (MSL) + (44.4m)     [paying attention to sign] 
Height (WGS84) = 599.3 metres 
Notes:

When supplying coordinates to others (for scientific research) it is suggested converting height to WGS84 (as shown above) is the best method to avoid confusion. The horizontal and vertical coordinates will then be in the SAME datum - which is more suitable for post processing. However, ALWAYS indicate what the horizontal and vertical datum is for supplied coordinates.

The horizontal datum on any GPS, can be set to any one of about a hundred "regional" datums. So people with hand-held GPS units, also need to check that the horizontal datum is set to WGS84 (default setting with KIWI OSD configuration file).

If the "geoidal separation" for a certain location is very close to zero, then it is hardly worth converting MSL to WGS84, unless the height values represent a very good "average" of many samples.

People in other countries, may need to use local maps or benchmarks to find their GPS vertical datum.

Some older GPS's ( Garmin 12XL and Garmin III+ ) have "bias" faults, which can confuse determination of vertical datum.

The Freeware Google Earth has a very nice feature, if you click the "Terrain box" on the main interface, you then get the height (MSL vertical datum) of where the mouse is. However as noted above, the height given by any map, has unknown precision - so use this feature with discretion - a GPS is more accurate (if the vertical datum is known)!

GPS receivers have a standard deviation of about 10 metres measuring height, so any one height reading should be within 10 metres of the TRUE height. To get better height accuracy, averaging values over a number of minutes will yield better results.

For those wanting to average GPS position (horizontal and vertical) here is a FREEWARE program I wrote that can do GPS Averaging, if one has a serial connection from PC to GPS.


Page written by Kiwi Geoff