Appendix

Curated list of additional resources¶

• Geospatial Transformations In-depth: ICESat-2 Hackweek 2022 tutorial by Tyler Sutterley, Hannah Besso and Scott Henderson that reviews CRS fundamentals and shows how to read CRS metadata and warp vector and raster data between CRSs.

• Applying Coordinate Transformations to Facilitate Data Comparison: NSIDC notebook that applies ITRF and plate-motion corrections so ICESat/GLAS, Operation IceBridge and ICESat-2 altimetry can be compared over 25+ years.

• UW Geospatial Data Analysis, Module 04: GeoPandas, CRS, Projections: David Shean’s course module introducing CRSs, projections and transformations, with GeoPandas exercises on ICESat data.

• Lost Without It: open GNSS textbook by Chris McAlister (University of Southern Queensland, 2023). Part 2, Datums and Coordinates explains ellipsoids, the geoid, datums and realizations from first principles, with examples drawn from Australian datums.

Specific to North America¶

• NGS Video Library: short videos from NOAA’s National Geodetic Survey and the COMET Program, including a four-part series on geodetic datums:

  • What are Geodetic Datums?

  • How Were Geodetic Datums Established?

  • What Is the Status of Today’s Geodetic Datums?

  • What’s Next for Geodetic Datums?

• Datums and Reference Frames: NGS overview of the official US datums, such as NAD83 and NAVD88, that make up the National Spatial Reference System (NSRS).

• New Datums: NGS hub for the modernized NSRS that will replace NAD83 and NAVD88, with timelines, FAQs and blueprint documents.

• Introduction to Geodesy and Maps: COMET MetEd module, developed for the US Navy, on geodesy, datums, map projections and map coordinate systems. Requires a free MetEd account.

• State Plane Coordinates: Washington State: Larry Signani’s 2007 refresher for the Washington State Land Surveyors Association, covering datums, ellipsoids, the geoid, map projections, and how State Plane grid coordinates relate to ground measurements.

• What is the difference between WGS84 and NAD83?: Short 2008 note by Mircea Neacsu (HYPACK) showing that the WGS84 and GRS80 ellipsoids are practically identical while the WGS84 and NAD83 datums have drifted about a meter apart.

What is a CRS?¶

• UW Geospatial Data Analysis, Module 04 demo: hands-on notebook that walks through CRS definitions and reprojection with GeoPandas.

• Common geospatial CRS definitions are recorded in the EPSG database and stored as WKT or JSON. OGC’s CRS-JSON-Encoding project is standardizing a JSON schema for CRSs, starting from PROJ’s PROJJSON.

Ellipsoid models¶

Transformations¶

• Allows you to go back and forth between different CRS

• You’ve all done this - convert from cartesian to polar coordinates (high school math)

• Can be 2D or 3D

• Most open-source packages depend on the PROJ library for CRS support and transformations

Vector¶

• pyproj: Python interface for PROJ

• GeoPandas uses pyproj under the hood for CRS transformations, with support for compound 3D CRS

Raster¶

• gdalwarp with proper CRS definitions (and available vertical offset grids) should work

Common datasets and CRS definitions¶

Raster DEMs¶

ArcticDEM/REMA/EarthDEM products from PGC¶

• These are distributed as 2D projected CRS (EPSG:3031 or EPSG:3413), without a vertical datum definition

• Elevation values are “height above the WGS84 ellipsoid” - but no details about specific realization used by vendor (Maxar) providing source stereo imagery

• Most of the Maxar data were acquired and delivered after 2008, so should be using more modern realizations of ITRF (2008, 2014, 2020), which are similar

• Can likely assume ITRF2014 for most of the available DEM products

• Can use custom WKT2 definitions for these 3D CRS:

  • Antarctica: ITRF2014_3031.wkt from SlideRule

  • Arctic: ITRF2014_3413.wkt from SlideRule

Copernicus 30 m DEM¶

• EPSG:9518 (WGS84 + EGM2008)

Point clouds and altimetry¶

3DEP lidar¶

• NAD83(2011) horizontal with NAVD88 vertical datum

• Projection is Local UTM Zone

• EPSG:6339+5703 (example for UTM Zone 10N with NAVD88 datum, see EPSG:6339)

  • Note: this is not the same as EPSG:32610 (WGS84) or EPSG:26910 (NAD83), because we are using NAD83(2011) realization

WA DNR¶

• EPSG:2926+5703 (WA state plane N)

• EPSG:2927+5703 (WA state plane S)

• NAVD88 model should be geoid2012 (I think, need to confirm)

Always check your dataset metadata!¶

• gdalinfo

• Review documentation, lidar reports, etc.

Testing and validation¶

VDatum¶

VDatum is NOAA’s free tool for transforming data between tidal, orthometric and ellipsoidal vertical datums.

cs2cs¶

echo -120.4 48.6 1400 | PROJ_DEBUG=2 PROJ_NETWORK=ON cs2cs -f "%.3f" -r epsg:7912 epsg:2927+5703

projinfo¶

projinfo -s EPSG:7912 -t EPSG:2927+5703 -o PROJ --hide-ballpark --spatial-test intersects

    Candidate operations found: 5
-------------------------------------
Operation No. 1:
unknown id, Inverse of Conversion from ITRF2014 (geocentric) to ITRF2014 (geog3D) + ITRF2014 to NAD83(2011) (1) + Inverse of Conversion from NAD83(2011) (geog3D) to NAD83(2011) (geocentric) + NAD83(2011) to NAVD88 height (3) + Inver
se of NAD83(HARN) to NAD83(2011) (NADCON5, CONUS) + SPCS83 Washington South zone (US Survey feet), 0.105 m, United States (USA) - CONUS onshore - Alabama; Arizona; Arkansas; California; Colorado; Connecticut; Delaware; Florida; Geor
gia; Idaho; Illinois; Indiana; Iowa; Kansas; Kentucky; Louisiana; Maine; Maryland; Massachusetts; Michigan; Minnesota; Mississippi; Missouri; Montana; Nebraska; Nevada; New Hampshire; New Jersey; New Mexico; New York; North Carolina
; North Dakota; Ohio; Oklahoma; Oregon; Pennsylvania; Rhode Island; South Carolina; South Dakota; Tennessee; Texas; Utah; Vermont; Virginia; Washington; West Virginia; Wisconsin; Wyoming.
PROJ string:
+proj=pipeline
  +step +proj=axisswap +order=2,1
  +step +proj=unitconvert +xy_in=deg +xy_out=rad
  +step +proj=cart +ellps=GRS80
  +step +proj=helmert +x=1.0053 +y=-1.90921 +z=-0.54157 +rx=0.02678138
        +ry=-0.00042027 +rz=0.01093206 +s=0.00036891 +dx=0.00079 +dy=-0.0006
        +dz=-0.00144 +drx=6.667e-05 +dry=-0.00075744 +drz=-5.133e-05
        +ds=-7.201e-05 +t_epoch=2010 +convention=coordinate_frame
  +step +inv +proj=cart +ellps=GRS80
  +step +inv +proj=vgridshift +grids=us_noaa_g2018u0.tif +multiplier=1
  +step +inv +proj=gridshift +no_z_transform
        +grids=us_noaa_nadcon5_nad83_2007_nad83_2011_conus.tif
  +step +inv +proj=gridshift +no_z_transform
        +grids=us_noaa_nadcon5_nad83_fbn_nad83_2007_conus.tif
  +step +inv +proj=gridshift +no_z_transform
        +grids=us_noaa_nadcon5_nad83_harn_nad83_fbn_conus.tif
  +step +proj=lcc +lat_0=45.3333333333333 +lon_0=-120.5 +lat_1=47.3333333333333
        +lat_2=45.8333333333333 +x_0=500000.0001016 +y_0=0 +ellps=GRS80
  +step +proj=unitconvert +xy_in=m +xy_out=us-ft

• Note the 0.105 m uncertainty

Gotchas and other notes¶

• There is no perfect transformation approach, and all transformations have some uncertainty

  • See Estimation of Vertical Uncertainties in VDatum for how NOAA quantifies the uncertainty of each transformation step

• There are many possible ways to go from one CRS to another, the PROJ pipelines allow you to control this

• Many CRS (esp compound or 3D CRS) don’t have EPSG codes - you can define the CRS with machine-readable, well-known text (use WKT2)