osgearth_conv generated TMSElevation dataset from ArcGIS not appearing properly

[michaelgtodd]

When using osgearth_conv to create a TMSElevation dataset from an ArcGISServerElevation dataset, and then loading that elevation data afterwards, strange artifacting is seen in the elevation data.

To Reproduce

1. Create a TMSElevation dataset

   osgearth_conv --in driver arcgisserverelevation \
             --in url https://services.arcgisonline.com/arcgis/rest/services/WorldElevation3D/Terrain3D/ImageServer \
             --out driver tmselevation \
             --out url /output_geo/tms.xml \
             --out format jpg \
             --min-level 0 \
             --max-level 3 \
             --extents -90 -180 90 180 \
             --profile global-geodetic \
             --no-overwrite \
             --threads 8

2. Create an earth file earthfile.txt

   
   <map name="ArcGIS Map">
   
   <ArcGISServerImage name="SAT">
       <url>http://services.arcgisonline.com/arcgis/rest/services/World_Imagery/MapServer/</url>
   </ArcGISServerImage>
   
   <TMSElevation name="ELEV">
       <url>/output_geo/tms.xml</url>
   </TMSElevation>

**Expected behavior**
The same elevation data from the online source is now available in the TMSElevation dataset

**Screenshots**
![image](https://github.com/gwaldron/osgearth/assets/794838/6350346e-3c72-4542-81ef-e676a6de0812)

**Environment**
 - OS: RHEL 8
 - OSGEarth Version 3.5

[TMwhuhan]

--out format tif ? save tms tile to tif image

[jasonbeverage]

@TMwhuhan is correct, you can't save elevation to jpg format as it will be quantized and scaled incorrectly. Try using --out format tif.