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FAQ: How do I apply a vertical datum transformation in ArcGIS Desktop?

Question

How do I apply a vertical datum transformation in ArcGIS Desktop?

Answer

Check the following items to facilitate applying vertical datum transformations in 10.4 and later with the ArcToolbox Project tool.

  • To apply a vertical datum transformation, the input data must be three-dimensional (3D). Features must be identified as PointZ or PointZM, PolylineZ or PolylineZM, or PolygonZ or PolygonZM in the Shape field in the attribute table.
    • Converting two-dimensional (2D) data with an elevation field into 3D can be accomplished with the Feature To 3D By Attribute tool. Using this tool requires a 3D Analyst license. In ArcToolbox, navigate to 3D Analyst > 3D Features > Feature To 3D By Attribute.
    • The elevation field must be a numeric field, and can be a Short or Long integer, Float, or Double.
  • An input vertical datum must be defined for the data. Use the 'Define Projection' tool in ArcToolbox to define the vertical coordinate system (VCS) for 3D data.
  • A horizontal geographic transformation and vertical datum transformation cannot be applied in the same Project tool process. These operations must be performed separately unless the "case" for the two projection/transformation processes is previously defined in the hvtdefaults.json file. Since this is difficult to determine, the recommended course is to perform the projection and transformation processes in multiple steps.
    • Project the data to the new horizontal coordinate system while applying the appropriate horizontal geographic transformation.
    • When the output data from the horizontal projection is created, open the Project tool and select the output as input. The output horizontal coordinate system must be specified for the output. Apply the vertical datum transformation in this step, and run the Project tool again.
  • There is no drop-down pick list to select the vertical datum transformations from at this time. A drop-down list is expected to be added at a future release of the software. In the meantime, the user must type the name of the vertical datum transformation that is to be applied, exactly as written in the list of transformations linked to the Knowledge article at the link below. If the vertical transformation will be run in the reverse direction, a tilde (~) must be added before the transformation name.
  • If the transformation is that of the vertical coordinate system, it is necessary to specify the output horizontal coordinate system as the same as the input. Even though the data is not being transformed to a different horizontal coordinate system, it is still necessary to specify the output coordinate system in the Project tool dialog.
  • At this time, if the vertical coordinate system for the data is defined in feet, such as NAVD88 USft (Height), there is no transformation available. The only available transformations are for meters-to-meters, and so no transformation appears in the drop-down if the input or output are indicated as feet. If needing to transform from NAVD88, which has units of meters, to NAVD88 USft (Height), the Project tool can be run without a transformation specified. The tool gives a warning, which can be ignored, and the units will be converted from meters to feet. Run the Add XY Coordinates tool on the output to add the heights in feet to the attribute table.

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