# Land Surveyor Community Forum

### GEOID ELEVATION

Hello,

I am a novice surveyor learning about the geoid and its used in calculating elevation.  In my research I see the reference made to H=h-N and recognize the two paths to a point, one from the ellipsoid representing "h" and one from the geoid representing "H" orthometric height.  It appears to me that the significance of the two paths is having a redundant calculation for a point P on the ground.   Is my understanding correct or is there a greater purpose in calculating elevations for two different paths as shown in the attached pic.

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#### Replies

• Exactly, when you certify an elevation you must specify the datum. Tidal bench marks are the most misunderstood. A tidal bench mark is for the most part, only good at the tide station where it was established. That is much of the reason modern datums were created. If you run elevations from a tidal bench mark a few miles down the beach to another tidal bench mark, it may not close. The problem gets worse if you run levels far inland.
• Thanks Charlie,

That makes sense.  It seems that the subject of tide datum versus geoid is a reflection of the development of the datums over time, that is, the current approach to favor use of the geoid and less use of local or Quebec tide datums ?

• My equipment is not quite as accurate as what NGS uses. I use a static GNSS base and OPUS solutions. Results can be very good. I have had NGS call me & ask if I had recovered monuments they assumed to be lost as my solution was
almost exactly as published for that monument. I was set up on it! Unfortunately, results are not always that good but elevation error is usually less than a couple CM. Tree cover severely limits the availability of good GPS sites & I don't have a rover, I therefore use GPS primarily to establish control. The OPUS report has pretty good indications of the accuracy in the report such as fixed ambiguity & overall RMS. Regular leveling is always a desirable check. There is usually data published for both NAD'88 & NGVD'29 for monuments close enough to use the data to apply conversions when needed. NAD'88 is almost obsolete & many FEMA FIRMS still use NGVD'29. I talk to the head surveyor at our State Geodetic Survey often & he suggested that I might improve my results by processing my own data. I try to check my equipment against a published mark on every job.
• Any program to Calc geoid from lat,long
• sometime i used benchmark which are established by differential leveling and refererence to MSL. In the project site like construction project, levelling is the appropriate way, this is more applicable. Apple to apple.

• If I am using differential leveling, does the NAVD88 Quebec tide datum represent the primary datum, or are there adjustments based on local tide datums typically ?

• Technically, you are working on a local datum. If we assume the benchmarks you are closing to are accurately described in the NAVD88 Quebec tudal datum then you a working in that datum.

JAC

• Mr. Turlington,

Redundant surveying observations are practically always beneficial.

The exact significance of your question is not clear to me.

The ellipsoid may be thought of like 3D graph paper. It is a man-made construct made as simple as possible for plotting positions on or near the earth.It is so simple it takes only two parameters to define it. There have been many attempts to define the ellipsoid over the centuries. After the advent of artificial satellites the knowledge of the shape of the earth has improved to the point that the last few iterations of refinement are miniscule. The most common ellipsoid in use today is GRS80.To be used as a datum a zero point for the longitude and the location of its origin  must be defined. Again the most frequent assignment is the Greenwich meridian and the center of mass of the Earth.

The geoid is a physical actuality. It is the surface of the points at which the gravitational potential is the strength of a reference - traditionally at or close to sea level. The geoid is not simple like the ellipsoid. It varies quite a bit from place to place depending on several factors, including latitude and the density of material under and around each point. It is not a mathematical surface.

What we did over the last few decades is to measure the gravity potential from place to place and developed something like a DEM of gravity potential - a geoid MODEL. The geoid model was not generally very useful until the popularity of GPS grew and its use for determining heights an elevations was desired.

GPS does not produce elevations. GPS gives excellent positions relative to the center of the earth and therefore, heights relative to the ellipsoid.The sole popular practical application of the geoid is to combine its model with GPS ellipsoid observations to approximate elevations (orthometric heights) by the formulae you illustrated.

I don't know if I addressed your interest, but I hope so.

JAC

• Mr. Cavell,

Thank you for your feedback, and yes, your response is of much help.  I thought the value of ellipsoid height “h” and the geoid undulations value "N" are both provided by GPS.  As a novice, I do not see the significance in needing the Orhtometric height "H" unless it is to provide two paths for finding elevation for purposes of checks and balances.  I see that Leveling would provide the elevation above MSL and thus the orthometric height using traditional methods.  I assume the survey leveling approach is more time consuming and it is much quicker to just use GPS to subtract the geoid undulations “N” from the ellipsoid height "h" to arrive at the Orthometric height, assuming it is needed.  Is it common practice to use leveling in addition to calculating orthometric height as H=h-N as a redundant check or is it just one method of using GPS over traditional methods of leveling?

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