CueTip – Using MHW Datums in GeoCue

GeoCue offers limited support for Mean High Water Tidal Datums. In GeoCue 2013, thirty-eight Mean High Water project areas (aka VDatum project areas released by NOAA on 13Aug2013) are supported and others will be added as they become available from NOAA. The twenty-five older VDatum project areas are also still included in the product. Transformation of data to a Mean High Water (MHW) datum is accomplished by creating a layer with its vertical coordinate system set to one of the VDatum project areas, and either populating, or copying geometry (e.g. Create Entity -> Selected Entities’ Geometry) from an existing layer.

The constraints for using these MHW datums are:

  1. You may only transform data “to” a Mean High Water datum. You cannot transform MHW data to any other defined vertical coordinate system.
  2. The vertical coordinate system that you are transforming “from” must be based on NAVD88. For example, if the input data has ellipsoidal heights, you must first do an intermediate ‘populate’ or ‘copy geometry’ step in GeoCue to convert your data to NAVD88 heights.
  3. All data that you wish to transform must be completely contained within the rectangular coverage of the VDatum project area to which you wish to transform.
  4. Any given project area will have “invalid regions” within it. Heights for points in these regions will be returned as -999,999, the value used by the VDatum utility. In the case of LIDAR point data, a macro step can be used to easily remove these heights from the LAS files.

As a consequence of the second constraint, any project for which you intend to transform data to a MHW datum should be created with its vertical coordinate system set to NAVD88. This is required because many transforms in GeoCue “go through” the project coordinate system.

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