update geoid conversion tutorial

Author: bakkerbakker

geoid_conversion/geoid_conversion.md

@@ -11,17 +11,23 @@ While the Earth is spherical, its surface is not a perfect sphere and is in fact
 
 PGC publishes its [DEM data products](https://www.pgc.umn.edu/data/elevation/)--ArcticDEM, the Reference Elevation Model of Antarctica (REMA), and EarthDEM--with a vertical reference of height above the WGS84 ellipsoid. Since these values can differ greatly from geoidal (orthometric) and Mean Sea Level heights, we will demonstrate how to convert the elevation values using GDAL command line tools and note some potential pitfalls to ensure you get the outputs you expect. 
 
-For the vertical reference conversion, we will use methods common for geospatial coordinate transformations using EPSG codes and append a vertical datum code when defining the target spatial reference system. While many countries have more accurate local geoid models, for the polar regions the best option is the [EGM08 geoid](https://epsg.io/3855) `EPSG:3855`. 
+For the vertical reference conversion, we will use methods common for geospatial coordinate transformations using EPSG codes and append a vertical datum code when defining the target spatial reference system. While many countries have more accurate local geoid models, for the polar regions the best option is the [EGM08 geoid](https://epsg.io/3855) `EPSG:3855`. The syntax to use with PGC's polar DEM products appends this vertical code to the EPSG code of the coordinate system for the GDAL command demonstrated below: 
+ - REMA (Antarctica): `EPSG:3031+3855`
+ - ArcticDEM: `EPSG:3413+3855`
 
 ### Sample conversion scripts
 #### Basic `gdalwarp` Command
 The basic command we will use is `gdalwarp` ([documentation](https://gdal.org/en/stable/programs/gdalwarp.html)), demonstrating a few options for PGC DEM inputs--local GeoTIFF file and AWS-hosted Cloud Optimized GeoTIFF (COG) on the cloud--and outputs--local GeoTIFF and VRT (Virtual Raster). Follow [instructions here](https://gdal.org/en/stable/download.html) to download and install GDAL. We recommend using Conda/Mamba to manage environments and installing with `conda install -c conda-forge gdal`. 
 
 The basic command structure is:
 
-```gdalwarp -optional_arguments -t_srs sourcefile destinationfile```. 
+```gdalwarp -optional_arguments -t_srs sourcefile destinationfile``` 
 
+ - Source File: input DEM
+ - Destination File: output DEM with geoid transformation applied 
  - Target Spatial Reference `-t_srs`: The optional argument you will need to do the vertical reference conversion is the `-t_srs` (target spatial reference system) using the `EPSG:XYproj+Zproj`. PGC DEMs are natively in polar projections, `EPSG:3031` for REMA and `EPSG:3413` for ArcticDEM. Adding the EPSG code for the EGM08 vertical reference `+3855` will convert ellipsoidal elevation values to height above geoid values. There is also an option to specify the source SRS `-s_srs`, but GDAL will read that data from the input file by default, so it is not necessary when using PGC DEMs. 
+
+Optional Arguments
  - Target Output File Type `-of`: You can also specify the output file type with the `-of`. GDAL will guess the output format from the file extension in the `dstfile` if none is specified. A useful output type is the Virtual Raster `.vrt`, which will not write the output to a new file, but will act as a pointer to the source file transforming the data on the fly when accessed. This is particularly handy to avoid essentially duplicating files when performing spatial transformations or merging raster files together.
  - Additional Creation Options `-co`: GDAL also provides additional creation options for each driver, including compression and tiling. Details can be found here for [GeoTIFF](https://gdal.org/en/stable/drivers/raster/gtiff.html#creation-options) and [COGs](https://gdal.org/en/stable/drivers/raster/cog.html#creation-options).   
 
@@ -51,6 +57,7 @@ gdalinfo dem_orthometric.tif
             AREA["World."],
             BBOX[-90,-180,90,180]],
         ID["EPSG",3855]]]
+...
 ```
 
 In QGIS, using an identify tool on the DEMs will return different elevation values for the original ellipsoid referenced DEM and the orthometric height DEM. Note that the .tif output and the equivalent .vrt output return the same values. However, the .vrt file includes downsampled overviews at larger zoom extents, which might return different values even though the underlying pixel values at full resolution are equivalent: