With the variety of drones, cameras, and flight planning applications available today, setting the correct vertical units and reference is vital to making sure your Drone2Map products align with other layers in the ArcGIS platform. Use the information and links below to better understand vertical datums and coordinate systems, the difference between a geoid and an ellipsoidal surface, and how to select the correct image and GCP vertical reference settings for your Drone2Map projects.
Understanding the following terms will help in choosing the correct vertical reference settings.
Vertical coordinate systems
A vertical coordinate system defines the reference for height values. As with a horizontal coordinate system, a vertical coordinate system ensures that data is spatially located accurately in relation to other data.
A vertical coordinate system can be referenced to two different types of surfaces: ellipsoidal (spheroidal) or orthometric (geoidal/gravity related). Most vertical coordinate systems are gravity related. Any vertical coordinate system can be used with different horizontal coordinate systems.
The geoid is an equipotential, or level, surface of the earth's gravity field. Imagine the oceans are allowed to settle under the influence of gravity only and are not subject to tidal or atmospheric forces, and tunnels are used to connect the oceans so that the water can move freely. The resulting surface is a representation of the geoid. The geoid is approximately equal to mean sea level (MSL) and generally differs from local mean sea level by a meter or so. It is a complex shape. The geoid is influenced by the composition of the earth. so it may have discontinuities in its slope. This means that the surface is an analytic surface as opposed to a mathematical surface such as an ellipsoid. The geoid generally differs from an earth-centered horizontal geodetic datum by less than 100 meters.
In the illustration above, the green line represents the geoid surface. It roughly curves to follow the topography. The dashed line represents the surface of the spheroid. The h is the height above the spheroid, or ellipsoid (HAE). In this case, the height is a negative value. Geoid undulation, N, is the distance between the spheroid and geoid surface. The orthometric height, H is related to the spheroid height by the following:
h = H + N
Vertical reference inputs
The vertical units and vertical reference settings are important properties in Drone2Map. In addition to an XY coordinate reference, your image locations and ground control point (GCP) locations each contain settings for vertical units and vertical reference. The combination of XY coordinate system, vertical units, and vertical reference determine the coordinates of Drone2Map output products.
Vertical (Z) units will almost always be the same as the units in the specified horizontal XY coordinate system. The only exception to this is when the horizontal coordinate system is a geographic coordinate system. In which case, vertical units will be meters.
In Drone2Map the following vertical units can be used:
- US survey feet
The vertical reference indicates the vertical model to which your elevation values refer. If you choose ellipsoidal, you have the option to provide a height above or below the applicable ellipsoid. This option is used to simulate other geoid models, such as Geoid12B, or to simply raise or lower the elevation of your Drone2Map output products. Note that the vertical reference choice may be different for images than it is for GCPs. You have the following vertical reference options:
- EGM 84—For altitudes based on the EGM84 geoid
- EGM 96—For altitudes based on the EGM96 geoid
- EGM 2008—For altitudes based on the EGM2008 geoid
- No Conversion
- Ellipsoid—For altitudes based on the ellipsoid specified in the XY coordinate system
EGM 84, EGM 96, and EGM 2008 are orthometric height models. Use these when your image or GCP elevation values represent height above the EGM84, EGM96, or EGM2008 geoid.
Set vertical reference to ellipsoid if your height values are in reference to the ellipsoid defined in the specified horizontal XY coordinate system of your images or GCPs. See the geoid section for a description of the relationship of geoid heights to ellipsoidal heights.
If you have GCPs, the height of Drone2Map products use the vertical units and vertical reference model used by the ground control. If you don't have GCPs, the output products are created using the vertical units and vertical reference model of the images.
If you have no GCPs and your images have an XY coordinate system of WGS84, Drone2Map output products are created using an XY coordinate system of UTM, meter as the vertical unit, and the vertical reference you specify for your images.
ArcGIS Online and ArcGIS Pro both use the EGM96 geoid height model. If you plan to publish a 3D mesh as a scene layer, you will want the 3D mesh product to have vertical units of meters and reference one of the three EGM geoid height models. Alternatively, you can provide a value for height above the ellipsoid to model a geoid surface such as Geoid12B, which closely approximates the EGM models. The goal is to have a 3D mesh that properly rests on the ground instead of sinking into or floating above the EGM96 surface.
Choosing a vertical reference
The vertical reference choice may differ for images and GCPs. Use these guidelines to help choose the vertical reference for images and GCPs.
The default vertical reference is EGM96 for images. Most image heights will be referenced to the EGM96 geoid and are either embedded in the exif header of the image or are contained in a separate file. Most GPS receivers convert the WGS84 ellipsoidal heights provided by global navigation satellites to EGM96 heights, so if you're unsure, accept the default of EGM96.
Choose No Conversion if your image heights are in reference to the ground, or if your project was flown with Real-time Kinematic system (RTK).
If your image heights are relative to the ground, check the settings in your mission planning software to see if there are options for referencing the EGM geoids or an ellipsoid instead. It is not advisable to use heights that are relative to ground.
The default vertical reference for GCPs is also EGM96. If your GCPs don't reference an EGM geoid height model, use ellipsoidal heights instead. If you want to raise or lower the GCP heights to model a geoid, enter the positive or negative offset between the ellipsoid height and the desired geoid at the location of your project. Your height units will always be in terms of the units of the horizontal (XY) coordinate system, so you may need to scale the height values accordingly.