California Surveyors Forums

For the stake of discussion. Do I need a goid to reduce for horizontal only and has anyone compared the results of horizontal using a goied vs. not? I need some thought proking discussion

This is test to see if Ian and Evan are working today.

I'm not sure I understand your question, Russell. The very nature of GPS measurement require that a geoid be employed to calculate a horizontal dimension. It would be inherently impossible to test the geoid v not scenario.

On the other hand, if you are discussing one particular geoid v another, you may get slight variations in your results. That's why we need to report the meta-data concerning the "measurement environment".

Elucidate, Grandpa Russ!

And I enjoy all five of the grandkids, and just got the news #6 will be here in 8 months.

But back to the dicussion, if only trying to measure relative to a local area without regard to SPC, is a geoid absolutely necessary?

The geoid model won't affect your horizontal distances. Ian, you might be confusing the geoid with the ellipsoid. Your gps points are stored as geodetic coordinates (actually vectors from your starting geodetic coordinate) with a height from the ellipsoid which is a perfect geometric model that somewhat approximates the earth's surface. The geoid model provides a height adjustment value based on your geodetic coordinates to more closely approximate sea level. I'm pretty sure that the geoid separation is not part of the equation for computing ground distances between geodetic coordinates, and even if it is it would only make a difference of one or two hundredths per mile. The elevation of your projection plane for your local coordinate system is more critical than the geoid model you use. I've seen some surveyors use a local coordinate system projected to elevation 0 when they were surveying at 5 or 6000 feet elevation. The distances in that case were all short about one foot per mile.

I suppose you could translate the Geodetic positions that you get using GPS into a local North, East and Up (as opposed to elevation) and then inverse to approximate a horizontal distance, but the question is why would you want to go thru all of that trouble for an approximation? With most GPS software and adjustment packages it is a simple thing to import the NGS geoid file. Why would you want to ignore that?

The State Plane Coordinate system also provides a good system to translate Geodetic positions to the mapping projection. By applying the proper grid to ground correction, you'll have a ground distance, and probably more accurate than you can measure without GPS.

Most GPS vendors are pretty good about supporting their customers. They want to make sure people think their equipment and software is easy to use. Check into the resources your vendor has to offer.

I wasn't working yesterday, but I am today, sort of. (Natasha just disputed whether posting on this forum can be considered work).

I don't know all that much about GPS, so I normally just read and learn from others more knowledgeable than I.

But I do know how to spell G-E-O-I-D. How's that for the 'stake' of discussion.

;-)