Define the Projection Ellipsoid to Match the Reference Datum

Recommendation Details: When producing geolocated image data derived from satellite-based or airborne remote sensing instruments, we recommend defining the projection ellipsoid to be the same as the datum used by the remote sensing system to define geodetic latitude and longitude.  Specific details depend on the selected file format and metadata conventions.  Examples provided in this recommendation use GeoTIFF terminology, but the recommendation also applies to other formats and metadata conventions.  For example, when using GeoTIFF to represent content in a Projected Coordinate Reference System (PCRS), the projection ellipsoid should be the same as the Geodetic Reference Frame (datum) used by the remote sensing system.  For many currently operating satellite instruments, the reported geolocation is referenced to the World Geodetic System (WGS) 1984 datum.  Airborne instruments that are geolocated using GPS instruments are also referenced to WGS84.  When geolocated data from one of these instruments are used to create derived geophysical products, data producers may choose a PCRS that includes a map projection based on a reference ellipsoid.  To ensure maximum interoperability when transforming such data products, we recommend choosing the PCRS map projection ellipsoid to match the underlying Geodetic Reference Frame (datum).  This will minimize potential for geolocation error with overlays of related geolocated information such as coastlines or comparison data products. 

In the following discussion, we use the ISO19111 terminology explained in Section 2.1.2 and Appendix A of Iliffe and Lott (2008):

Image transformation of projected data products may require a coordinate conversion or a coordinate transformation.  A coordinate conversion is a change of coordinates from one Coordinate Reference System (CRS) to another.  The CRSs can be based on the same datum, or, if the datums are different, no algorithm is applied to transform the coordinates of one datum to the other.  Iliffe and Lott (2008) contend that, since coordinate conversions are considered to be exact, there is no loss of positional accuracy when a coordinate conversion is performed without transforming differing datum coordinates.  A coordinate transformation is a change of coordinates from one CRS to another, in which the CRSs are based on different datums.  In this case, a coordinate transformation algorithm is applied to convert the coordinates of one CRS to conform to the datum of the other CRS.  (Further discussion and case study example are included in Section 2. of Brodzik et al. 2012).

It is possible to properly encode both the PCRS map projection ellipsoid and the Geodetic Reference Frame datum in, for example, GeoTIFF metadata.  However, some software packages may either incorrectly assume they are the same, or require that they be the same, in order to perform accurate coordinate conversions.  Depending on spatial resolution of the image content, the effects of performing an incorrect conversion and/or transformation may not be visually apparent, or may only be apparent if the data include an obvious feature like a subtly shifted coastline when overlaid with independently-derived coastline vectors.  A detailed example depicting NASA Operation Ice Bridge flightlines on incorrectly transformed NASA Blue Marble imagery is included in Brodzik et al. (2012).

Defining the PCRS map projection ellipsoid to match the underlying Geodetic Reference Frame ensures that software packages making this assumption will do the right thing, and will eliminate the time that users might otherwise have to spend to direct the software to only perform the requisite coordinate conversion or coordinate transformation.

In the case of GeoTIFF, this recommendation may become obsolete, given the more specific user-defined details articulated in the OGC GeoTIFF Standard v1.1 (2019).  We note that section B.2.3 of this document explicitly acknowledges historical examples that have used a spherical projection ellipsoid, but discourages the use of spheroids for modern applications.  Given the recent date of this standard, it remains to be seen in practice how closely software packages adhere to encoded projection ellipsoids and reference datums when performing coordinate conversions and transformations.

References:

Brodzik, M. J., B. Billingsley, T. Haran, B. Raup, M. H. Savoie. 2012. EASE-Grid 2.0: Incremental but Significant Improvements for Earth-Gridded Data Sets. ISPRS International Journal of Geo-Information, 1(1):32-45, doi:10.3390/ijgi1010032.

Iliffe, J. and Lott, R. Datums and Map Projections for Remote Sensing, GIS and Surveying, 2nd ed.; Whittles Publishing: Scotland, UK, 2008.

OGC, OGC GeoTIFF Standard, v1.1, OGC Document 19-008r4, September 14, 2019. http://docs.opengeospatial.org/is/19-008r4/19-008r4.html