update links on repo readme for geoid conversion

Author: bakkerbakker

README.md

@@ -14,4 +14,7 @@ To jump in directly, the dynamic STAC API link is: [https://stac.pgc.umn.edu/api
 ### The Digital Elevation Model (DEM) data
 The STAC API provides access to the public DEM data that PGC produces from high resolution satellite imagery, specifically for the polar regions through the [ArcticDEM](https://www.pgc.umn.edu/data/arcticdem/) and [Reference Elevation Model of Antarctica (REMA)](https://www.pgc.umn.edu/data/rema/) products, as well as a subset of the [EarthDEM](https://www.pgc.umn.edu/data/earthdem/) data for the area around the Great Lakes region. These are continent-scale, high resolution (2m), repeat coverage elevation models published as time-stamped DEM Strips and (mostly) seamless Mosaics. You can find out more about [this elevation data on our website](https://www.pgc.umn.edu/data/elevation/).
 
+## DEM Vertical Reference Transformation
+### Ellipsoid to Orthometric (geoid) elevation conversion
+PGC's DEM products are published with elevation values relative to the WGS84 Ellipsoid. If users need orthometric elevation values, where zero at the Mean Sea Level, they will need to perform a vertical reference transformation. [This tutorial](/geoid_conversion/geoid_conversion.md) describes the process and provides sample commands to convert the elevation values to the appropriate vertical reference system.