Canadian (Ontario) High-Resolution Aerial Maps WMS - Troubleshooting

William Ellwood
I was inspired by the helpful thread on loading Canadian CHS nautical charts into CalTopo (https://help.caltopo.com/hc/en-us/community/posts/10865622699803), and I've been trying to do the same with Ontario's free high-resolution aerial imagery and wanted to share what I've found and where I'm still stuck.
 
Ontario's Ministry of Natural Resources publishes GeoHub — a publicly accessible ArcGIS ImageServer with nice aerial orthophotography:
- 20 cm/pixel resolution
- All of southern Ontario — roughly Windsor to Ottawa
- No account, no API key, no token required for reasonable usage as far as I can tell

 

Service endpoints:



What I've tried so far:
1. Auto-configure via WMS URL in CalTopo - CalTopo auto-populated `LAYERS=0`, which is the non-queryable root container in the ArcGIS WMS structure. The service returns `InvalidLayers` for it. I think that the correct layer name for visible RGB is `GEO_Imagery_Data_Service_2023to2027:SCOOP_2023`, but I'm not sure if this is being picked up correctly.
 
2. Manual WMS GetMap URL with the correct layer name - works perfectly in a browser and QGIS (confirmed returning full PNGs), but in CalTopo's Custom Source > WMS type it loads without error but without and loaded imagery.

 

Following Marc Chauvin's pattern from the CHS thread I tried using the REST exportImage endpoint directly in CalTopo's WMS type, with `{left},{bottom},{right},{top}` and `{tilesize}` placeholders:
 
 
One gotcha discovered: `bboxSR=EPSG:3857` (the string form) returns a blank 111-byte image from this service. Using the numeric WKID `bboxSR=3857` returns the correct full image (555 KB at 512×512). The REST endpoint is confirmed working via curl and Python — it's just not displaying in CalTopo.
 
The CHS solution in the referenced thread uses a MapServer `/export` endpoint. This service is an ImageServer using `/exportImage`. I'm wondering if there's a parameter difference CalTopo needs, or if the ImageServer variant requires a different approach entirely.
 
Any help or pointers appreciated; and if anyone gets it working, I'd love to know the exact URL format. It seems like it's running the same software that is supporting the WMS that is referenced in the Custom Layer docs, but I'm new to this and I'm obviously missing some things.

 

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