Conversion Tools for Transforming Geode Data

16 Jan 2024

Taking precise measurements of locations on Earth is complicated. The following information about the specific way that the Geode provides precise measurements is provided to assist those who need to convert or transform data.

Geoid

According to NOAA, "The geoid is a model of global mean sea level that is used to measure precise surface elevations."¹

The default geoid used by the Geode is a modified version of the Defense Mapping Agency (DMA) 10-degree x 10-degree 1974 grid model. The modification reduces the number of sample points in the polar regions to remove unnecessary oversampling that would otherwise be present. This modified model is not publicly published and cannot be downloaded. For most Geode users, the impact of this modification is negligible. If you are using the Geode in polar regions (above 80 degrees), the Geode data for geoid height offset would vary slightly from the original DMA 10x10 model.

To transform the reported Geode elevation to a different geoid model, you need the original ellipsoid height. To calculate the ellipsoid height you need both the orthometric height (elevation) and the geoid height offset for the points captured. Algebraically add the geoid height offset to the orthometric height to get the ellipsoidal height.

Orthometric Height + Geoid Height Offset = Ellipsoid Height

Once you have calculated the ellipsoid heights for your data, you can then use one of many third-party conversion tools to calculate new orthometric heights from known published models. For example, in the United States, you could use Vdatum² provided by NOAA.

Datums

According to NOAA, "A geodetic datum is an abstract coordinate system with a reference surface (such as sea level) that serves to provide known locations to begin surveys and create maps...There are two main datums in the United States. Horizontal datums measure positions (latitude and longitude) on the surface of the Earth while vertical datums are used to measure land elevations and water depths."³ There are two official datums in the United States horizontal (NAD83) and vertical (NAVD88). A vertical datum will reference a geoid when determining its reference surface.

In the United States, the formal land measurement datum used in official records is the North American Datum 1983 (NAD83). States define their local coordinate system based on this datum.⁴ Datums are updated regularly to reflect changes in the way they are calculated as well as changes in the shape of the earth. The updates, adjustments and epochs, are noted with the specific time they are defined. For example, NAD83(2011) epoch (2010.00) indicates the 2011 datum adjustment and the published coordinates of the National Geodetic Survey (NGS) Continuously Operating Reference Stations (CORS) on January 1, 2010. (The epoch date is given in decimal years.) 5 The current epoch includes updates that address the periodic tectonic plate motion of the Earth. The updates keep the Prime Meridian (0 degrees Longitude) at Greenwich, England, even though the entire crust of the Earth is moving.

When performing a coordinate conversion or transformation it is important to apply the correct datum model including the specific adjustment and epoch because the models change over time.

Transforming between Coordinate Systems and Geoids

The datum and coordinate reference system used by the Geode will vary with the correction being used.

If you are using an NTRIP/RTK/CORS network source, the datum will likely be an iteration of NAD83, such as NAD83(2011) epoch(2010.00).
If you are using an SBAS/WAAS or Atlas correction, the datum used will be based on the International Terrestrial Reference Frame (ITRF)6, a realization of WGS84.
- With WAAS correction employed the Geode uses the ITRF model, which is based on the current epoch of WGS84.

Transforming between WGS84 and NAD83 is not simple. There is no direct relationship between WGS84 and NAD83. They are based on different models of how to measure location on the Earth and serve different purposes.

	Underlying Model	Relation to Plate Tectonics	Managing Agency
NAD83	Plane Coordinate System—perpendicular east-west, north-south grid	Tied to static continental US references (North American Plate)	National Geodetic Survey
WGS84	3-Dimensional Coordinate Reference Frame—origin and orientation of coordinate axes	Moves with global plate tectonics and has regular adjustments when the global model gets more than +1cm off	National Geospatial-Intelligence Agency (NGA)

Conversion Tools

A popular program and development kit that can be used to perform conversions is called PROJ. Along with PROJ, you could use code from NGS for their HTDP (Horizontal Time-Dependent Positioning) app. Using these tools, you can transform data between various NAD83 and WGS84 realizations.

Horizontal Time-Dependent Positioning (HTDP) converts WGS-84 datum based data (SBAS/WAAS or Atlas Correction Source) to NAD83.

https://vdatum.noaa.gov/docs/services.html#step150 is more complicated than other NGS tools but works well.

If you export data as KML or SHP, then the following conversion tools may work.