Sometimes in surveying, people refer to the same point on the earth's surface, but they get different elevations. How is that possible? The reason is because each person measured the same point's elevation through a different elevation reference (or vertical datum). This article gives an explanation regarding the different elevation references, along with a quick explanation regarding vertical datums and their proper set up.
Essentially, we have 3 main categories of elevation references:
- Ellipsoidal Height
- Geoid Height
- Orthometric Height
The Ellipsoidal Height (h) is the difference of the vertical distance between a point on the Earth's Surface and the ellipsoid. The Ellipsoidal height is also known as the geodetic height and should not be confused with geodetic datums. When capturing coordinates with a GPS receiver, the elevation data references the ellipsoid, which means each captured coordinate needs to be calculated to match elevations with the more accurate geoid height.
The Geoid Height (N) is the difference of the vertical distance between the reference geoid and the ellipsoid. The geoid is a hypothetical shape of the earth that often coincides with the average of the earth's sea level and its imagined extension above or below land areas; the geoid height may sometimes be referred to as the elevation at Mean Sea Level (MSL).
The Orthometric Height (H)—the height we ultimately want to know—is the difference between the vertical distance from a location on the Earth's Surface and the geoid. Because the geoid coincides with MSL, whenever you see elevation data described as "X" feet above or below sea level, it is referring to the orthometric height.
Local Vertical Datum
There are several realizations of local vertical datums in the world. They are mostly parallel to the geoid but are often shifted vertically by a couple of meters. This offset is due to local phenomena such as ocean currents, tides, coastal winds, water temperature, and salinity at the location of the tide-gauge.
For example, Belgium uses Tweede Algemene Waterpassing (TAW) while its neighboring country, the Netherlands, uses Nieuw Amsterdams Peil (NAP). This often creates confusion when doing projects that cross over international borders. Because the elevation is measured with respect to a different local vertical datum, the elevation numbers between different countries can differ.
Where to Set the Proper Vertical Datum?
The proper elevation reference level (or datum) needs to be set up during the photogrammetry process. Processing the photogrammetry is a necessary step before you can start working in Virtual Surveyor. See an overview of the complete workflow here on how Virtual Surveyor relates to photogrammetry, along with the entire drone survey workflow.
Virtual Surveyor offers a quick alternative to adjust the elevation datum to a different one, in the event your photogrammetry was set up with an improper datum. You can read more about how to offset the Elevation Terrain using the Offset Z function, which can be found in the TOOLS for Elevation Terrain tab when the Elevation Terrain is selected in the Project View.