gps geodetic values to planar elevations

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gps geodetic values to planar elevations

Postby mlbrack » Mon Aug 11, 2008 6:29 pm

I'm stumped on the easy way to do this. I can work flat and I can work curved, but how to I put the two together?

I'm tied into a gps network. H is based on gps values, V is derived from leveling. How do I get my gps elevation to jive with the level?

Not sure the right, easist way to deal with this problem. I shouldn't be using a geoid model but rather should be using a constant value for a geoid separation?

A grid to local transformation will only average out the difference between geoid values and the level control values? That is the difference will be spread out over the control net.

Point me in the right direction, ya'll.

Mike
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Postby Jason Poitras » Mon Aug 11, 2008 10:15 pm

Hi Mike,

Four questions:

1) How large of an area are we talking about?

2) How far apart are the control points that you have orthometric (leveled) heights for?

3) Have these points been located with GPS?

4) Do you know what the geoid seperation is at each point?

After reviewing your answers, I'll post a reply.
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Postby mlbrack » Tue Aug 12, 2008 6:37 am

Thanks for all the good info on multiple posts, Jason.

To answer your questions:

1) the total project area is about 800m x 800 m

2) vectors between controls range from about 200 to 400 m

3) yes, a static gps control net has been established; I am tying to someone else's control net and have established two additional gps antenna mounts that I have used static runs to tie to the net

4)yes, I can get the geoid separation for all the controls

The area is almost flat and so far it looks like about 2 cm max difference between geoid and leveled heights.
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Postby Jason Poitras » Tue Aug 12, 2008 8:17 pm

Very good question.

Option 1

If you take the leveled height, minus the GPS derived ellipsoid height, how does the difference compare to the geoid separations for the points? It should be close. If they compare very closely you can probably get away with computing an average value that will take you from ellipsoid heights to leveled heights. For the site you could take an average of the difference between ellipsoid heights and leveled heights and input that into the elevation field in the transformation settings screen. Just make sure you set the vertical datum to ellipsoid.

Option 2

The other thing you can do is setup the control points in FieldGenius so that you have the leveled heights defined for them, and set the vertical datum to a geoid, such as NAVD88. Tie in the control points with GPS and on the control page for each point turn off the "hold horizontal", and leave on the "hold vertical" toggles. This will solve for only the vertical positions and will end up computing an elevation difference between the geoid, and your leveled heights. The good thing about doing this is you will be able to see the delta difference in the heights at each point in the transformation settings screen.

With option 2, you can also experiment with using the "Do not calculate vertical slopes" checkbox on the Edit Control page. If this is on then FieldGenius will not tilt the plane to best fit the control points. Sometimes tilting the plane can give you better results which can be confirmed by observing the elevation deltas.

I hope this helps answer your question.
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Postby mlbrack » Wed Aug 13, 2008 6:10 am

Yeah, that helps.

What you've outlined is basically how I was thinking about it, but you've definitely saved me some fumbling around.

Thanks again, Jason.
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GEOID SEPARATION

Postby Richard Sands » Wed Aug 13, 2008 5:07 pm

Mike, not sure what the separation is, but another thought - can you create a model (surface) of the separation differences, another model (surface) of the Levelled levels. Then you can take the 1st from the 2nd.
I realise its a bit involved but not really sure what you'r trying to achieve.
Once the Geoid model is set up then thats it - no more messing about.
I assume you are working to fairly fine tolerances over the area and the Levelled 'levels' are up to those standards?
I'm not sure if you are looking at RTK stuff or post process the actual results.
regards
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