It's true that elevation from GPS receivers are measured from the ellipsoid as a reference rather than the geiod/mean sea level (MSL). This topic is a little trick though.
But my question is whether there are specialized GPS receivers that are capable of doing the conversion from ellipsoid to orthormetric and hence output othormetric elevation. Or is there any way any GPS receiver can be tweaked to do this calculation automatically?
GPS as system calculates Altitude (height over ellipsiod) and by adding the undualation part it becomes height (height over mean sea level).
Almost all good receivers provide both.
In NMEA msg GGA, height is provided as above mean sea level
However the altitude (hieght over ellipsoid) is provided in the device specific messages or binary message. e.g. SiRF provide altitude in PSRF104.
GNSS receivers can directly output orthometric or ellipsoidal heights. It depends only to the vertical reference you use. If local geoid is used in the GNSS receiver, It is clear that you will approximately have orthometric height. If the vertical reference used is the ellipsoid as WGS84 you will has ellipsoid height.
Thanks Dr. Abdull Malik for your quick response to my question. Would it be correct to say that the figures we read off GPS as elevations are in MSL, or are they still height above the ellipsoid. I will like to categorically know if possible.
Thanks again in advance
Thanks Yap Loudi for your response.
I was wondering if local geoid or the global geoid ( Geoid99 ) is a compulsory component of the GPS receiver, or would I need to acquire this differently? Actually, the intent is to get to know a way of getting my GPS elevation readings in MSL. Thanks in advance.
dear Elijah, All the GPS receivers provide the altimetric component above the ellipsoid. If you want to have the orthometric height you will inroduce a grid of a local geoid determined by using global geopotential model, gravimetric data and DTM on and outside the area.
Partially yes.
As I understood from your question that you want direct othormetric elevation. So you can ask the GPS Receiver Manufacturer to set the internal programme of the device as per your recommendation because it is nun but mathematics & programming or else you can develop a programme by your self for the conversation in Python/R domain.
All the GNSS receiver measure the Altitude (hieght over ellipsoid) but in the NMEA $GPGGA message they add the undulation based on the geoidal model selected within the receiver, but usually it is based on the official NATO's standard mean-sea-level algorithm. The GGA message also reports the calculated undulation value from which some guidance about model used can be taken. for GGA message you can see the reference
http://home.mira.net/~gnb/gps/nmea.html#gpgga
for knowing the actual geoid model use the GPS manufacturer supplied software.
GPS derives ellipsoidal height, unless your receivers were installed with geoid heights of your area.
GPS/GNSS receivers do not give orthometric heights, they only give ellipsoidal heights. This is because, the heights are geometric as they are computed with respect to the ellipsoid. Orthometric heights are obtained with respect to the geoid/MSL. Orthometric heights can only be obtained from processed GPS/GNSS observations if the local geoid model of the area of observation is applied.