FAQ: Heights, Z-values, and vertical datums in ArcGIS


With the integration of 2D Maps and 3D Scenes in ArcGIS Pro, coupled with broader use of high-accuracy GNSS, there is a growing need to consider vertical datums when working with GIS data. This blog seeks to unpack some key ideas around vertical datums within an Australian context, as well as provide some example workflows you might utilise when working with z-enabled data.

Put simply, a vertical datum defines the origin for height or depth values in ArcGIS. Let’s first look into some key terms which we might see when working with vertical datums – ideas which are discussed in further detail here:

Ellipsoid and Spheroid

               These terms are used interchangeably in the geospatial community; however, the spheroid is the preferred term in Esri documentation and will be used in this article. A spheroid is a mathematical representation of the earth, selected for use in a specific geographic area. This representation is based on a sphere, with a flattening applied to represent the ‘bulge’ in the earth at the equator.

Datum

               A datum incorporates the spheroid and provides parameters that define the origin of the spheroid with reference to the earth. If the spheroid is the ‘size and shape’, then the datum is its ‘position’.

Geoid (and AusGeoid)

               A geoid is a complex surface applied to the spheroid to provide a better approximation of heights on the surface of the earth. The geoid-corrected heights are often referred to as orthometric heights. A geoid allows for the calculation of orthometric heights by applying the geoid-ellipsoid separation (or N) to the spheroid height (see diagram below)

We use AusGeoid2020 to better approximate orthometric elevations in Australia for Mean Sea Level or Australian Height Datum (AHD). AusGeoid2020 is currently the best determination of AHD from GDA2020.

Vertical Datums and Transformation in ArcGIS – FAQ

Why do we need vertical datums and transformations?

When working with z-enabled data describing elevations, it’s necessary to define the datum so that others’ working with the data are aware of the reference point of those elevation values. Transformations are required because they provide an avenue to move between different datums. For instance, you may be provided with spheroid elevations in GDA2020 and may need to work on a project in AHD, referencing mean sea level in Australia. A vertical transformation using AusGeoid2020 allows you to transform these values from GDA2020 to AHD.

What is the coverage and accuracy of AusGeoid2020?

AusGeoid2020 has larger extents than the previous geoid, AusGeoid09, and now provides coverage of Australia and Indian Island Territories (Christmas Island, and Cocos and Keeling Islands). Esri’s stated accuracy of the vertical transformation from GDA2020 to AHD using AusGeoid2020 is 0.03m. It should be noted that the geoid has some distortions and biases which should be taken into consideration before applying this accuracy stated by Esri (more on that in this link).

How does ArcGIS Pro support vertical transformations to AHD?

A native install of ArcGIS Pro 2.8 does not include the AusGeoid2020 transformation. There is a vertical transformation in Pro (GDA2020_Height_To_AHD_Height_AUSGeoid2020 _1) which is included in the ‘ArcGIS Coordinate Systems Data’, the instructions to download this are included here.

Why are AHD and AHD Tasmania separate in the vertical datums list?

The leveling network to determine AHD was conducted on Mainland Australia independently of Tasmania. Therefore, there are two vertical datums available with the relevant extents for their area of coverage. The GDA2020_Height_To_AHD_Height_AUSGeoid2020 _1 transformation covers and will transform from GDA2020 to AHD within Tasmania, Mainland, and Island Territories. More information here.

Is the Australian Vertical Working Surface (AVWS) supported?

The Australian Vertical Working Surface (AVWS) is a new reference surface for heights in Australia. From ArcGIS Pro 2.9, the new Australian Vertical Working Surface is supported as a vertical datum. The new transformation GDA2020_To_AVWS_Height_2 provides avenues for transforming existing data to and from AVWS.

How do I transform between Spheroid/GDA2020 and orthometric/AHD in ArcGIS Pro?

Provided that you have correctly defined the vertical datum when creating or importing data in ArcGIS Pro, you can change the (vertical) Map Coordinate System and transformation to alter the height format used in the Map Frame. The vertical datum work in much the same way as the ‘on-the-fly transformation’ in the horizontal coordinates systems.

With this in mind, In ArcGIS Pro, we have two potential workflows.

Method 1: Transform On-the-fly: Take this example of a layer that was collected using Field Maps in GDA2020.

  1. Checking the layer properties in ArcGIS Pro, see that the vertical datum is GDA2020.
  2. In Map Properties (right-click) set the Map ‘current Z’ value to AHD.
  3. Specify the transformation (GDA2020_Height_To_AHD_Height_AUSGeoid2020 _1). Pressing OK will allow for on-the-fly transformation of the z values.
  4. Select a point, then select attributes -> geometry. The z value which is showing has now been corrected by the geoid-ellipsoid separation value and represents AHD.

Note: in this workflow, we only change the coordinate display, not the underlying coordinates stored in the layer.

Method 2: Reproject the layer: Note that we use the layer from the example above, as it still has the GDA2020 vertical datum applied

  1. Open the ‘Project’ geoprocessing tool to create a new layer with the vertical datum set to AHD.
  2. Select the input dataset or feature class, and output name.
  3. For the output coordinate system, select the vertical datum for the output layer to be created in (in this case, AHD).
  4. Check the ‘vertical‘  box to tell ArcGIS Pro to apply a vertical transformation to the coordinates.
  5. Specify the transformation path to link GDA2020 to AHD using the geoid (GDA2020_Height_To_AHD_Height_AUSGeoid2020 _1).
  6. Press Run.
  7. Examine the map properties to see that no on-the-fly transformation is applied, as the Map and Layer both exist in AHD.
  8. Select a point, then select attributes -> geometry. The z value which is showing has now been corrected by the geoid-ellipsoid separation value and represents AHD.

The ‘vertical’ checkbox in Project or Project Raster has an important function. If the box is unchecked, which is the default. The vertical datum will be written to the output dataset, but the data will not be transformed (a null transformation). However, with the ‘vertical’ box checked, the z values of the output data will be transformed using the selected transformation.

Can we just define projection to specify the vertical datum? Where does this fit in?

If the data has been added to a z-enabled layer without an unknown projection, you will need to use the define projection tool. If you run the Define Projection tool to AHD on a layer with GDA2020 elevations, you are not changing the underlying elevations, you’re just specifying the vertical datum of the data.

What about elevations in ArcGIS Online, or Portal for ArcGIS?

The scene viewer supports vertical datums. If you are sharing to ArcGIS Online or Portal for ArcGIS, the vertical datum will be taken from the scene properties and carried through to the published scene. There is an excellent blog that goes into this in more detail here. 

What do I do in Field Maps?

If working with data in Field Maps, you do need an awareness of the vertical datum and geoid that your receiver is using, as it will help you make decisions regarding vertical datums as you complete your fieldwork and processing workflow.

The way Field Maps records positions is from reading an NMEA stream of positions from the GNSS receiver interface (directly from your phone, Trimble Catalyst, Leica Zeno, or similar), the message contains both the Orthometric Height and the Geoid-Ellipsoid Separation which are read by Field Maps to deduce the elevation. See the example below.

A sample NMEA GGA message for Bendigo, Victoria.

$GNGGA,22437.77,3615.173353,S,13415.687886,E,1,12,0.6,250.2,M,3.9,M,,*59

250.2 is the Orthometric Height

3.9 is the Geoid-Ellipsoid Separation.

Field Maps currently only supports horizontal transformations, so the data collected should be in AHD (by applying a geoid in your GNSS receiver), or, you’ll need to collect in GDA2020, and apply the transformation after data collection at the layer level in ArcGIS Pro.

A potential issue is that most phones use EGM96, a global Geoid, rather than AusGeoid2020 – which is not critically important for some users, because the accuracy of the vertical position in a mobile phone is usually less than the difference between EGM96 compared to AusGeoid2020, but it’s something to be aware of as mobile devices continue to improve positional accuracy.

What about rasters?

Vertical datums and transformations apply to raster data too. The workflows are similar to what is described with vector data, however, a different Geoprocessing Tool, Project Raster, must be used to transform raster data between vertical datums.

Other resources:

GDA2020 and Ausgeoid2020 are just components of continually evolving methods for describing elevations in Australia. The following articles provide further information on the latest advances in datum modernisation.

https://www.icsm.gov.au/australian-height-datum

https://www.icsm.gov.au/australian-vertical-working-surface

ArcGIS Pro documentation on vertical coordinate systems: https://pro.arcgis.com/en/pro-app/latest/help/mapping/properties/vertical-coordinate-systems.htm

List of supported Vertical Coordinate Systems in ArcGIS Pro: https://pro.arcgis.com/en/pro-app/latest/help/mapping/properties/pdf/geographic_coordinate_systems.pdf

List of supported Vertical Transformations in ArcGIS Pro: https://pro.arcgis.com/en/pro-app/latest/help/mapping/properties/pdf/geographic_transformations.pdf

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