Apply datum transformations when collecting data with ArcGIS Field Maps

Description

This article provides two different examples of settings in the location profile for ArcGIS Field Maps. The location profile applies datum transformations between the basemap and the correction service used. This improves accuracy for Global Navigation Satellite Systems (GNSS) correction in the United States. Different transformations may be necessary depending on the output from the GNSS services available in other countries.

In Field Maps the user sets the coordinate system of the receiver to the coordinate system of the basemap on which data is collected. This may be any datum in use in the United States, including NAD 1927, NAD 1983, or variations such as NAD 1983 HARN, NAD 1983 NSRS 2007, NAD 1983 CORS96, NAD 1983 2011, or WGS 1984.

The correction services below provide data on the datums indicated. Datum transformations must be applied to adjust between the receiver coordinate system and the correction service below.

• RTK: Real-time kinematic positioning, which is the application of surveying to correct for common errors in current satellite navigation (GNSS) systems. This system returns coordinates on GCS_NAD_1983_(2011).
• SBAS: Satellite-based augmentation systems, which are regional, also improve data collection accuracy and provide data on GCS_ITRF_2008.

Cause

Both validators may require more than one geographic (datum) transformation to align the collected data with the geographic coordinate system of the basemap. These must be entered into the location profile when setting up Field Maps.

Field Maps can only process two geographic transformations in the location profile. However, in some cases, more than two geographic transformations may be required depending on the correction service used and the datum of the user's basemaps.

Output data will not line up if a third transformation is required, but there may be a workaround available, as detailed below.

Solution or Workaround

Applying a correction using RTK

Suppose the user is working with RTK, which outputs coordinates in GCS_NAD_1983_(2011), and the basemap is in WGS_1984_Web_Mercator_Auxiliary_Sphere. In the location profile, the GNSS coordinate system is GCS_NAD_1983_2011. When using an Esri basemap, the map coordinate system is WGS_1984_Web_Mercator_Auxiliary_Sphere. The top selection that appears in the transformation dialog is WGS_1984_(ITRF08)_to NAD_1983_(2011) (WKID 108363), which is correct.

Applying a sample correction using SBAS

Using SBAS is more complex because the input is GCS_ITRF_2008. To transform from ITRF_2008 to the original NAD_1983 datum used for the basemap in this example requires the following sequence of three coordinate frame transformations. The names in this chart refer to the "forward" direction for each transformation.

Use the following workaround instead of trying to apply three transformation in Field Maps.

ITRF 2008 incorporates position changes due to continental drift based on a specific date called the epoch. The original NAD 1983 datum did not include those changes because, in the mid-1980s, when this transformation was calculated and published, instrumentation was not sensitive enough to track such miniscule continental movements. Therefore, it is necessary to work backward from the current data collection to older, less accurate data positions in order to align the data being collected on the older datum.

Datum transformations work in either direction with equal accuracy. In the chart above, you see the parameters for WKID 108360 and WKID 108363 are identical but the first transformations is run forward while the second transformation is run in reverse. Because the transformation parameters are identical, the transformations cancel each other out; they do not change the coordinates of the data. They only rename the datum from ITRF 2008 to NAD 1983 (2011) to WGS 1984.

In this example, because the first and second transformations are identical, the user can eliminate both of them. Instead they can substitute an additional transformation with zero for all parameters; this renames the datum from ITRF_2000_To_WGS_1984 (WKID 108280) to WGS_1984_(ITRF00)_To_NAD_1983 (WKID 1081900). ITRF 2000 can substitute for ITRF 2008 because the difference between them is only 0.8 mm (0.1 mm per year).

For more information on supported geographic transformations, refer to How To: Select the correct geographic (datum) transformation when projecting between datums.