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How to get the data out of an ArcGIS web map

The short answer: find the /rest/services/.../FeatureServer/0 URL in the network tab, then append /query?where=1=1&outFields=*&f=geojson to download the features. If it says MapServer instead of FeatureServer, you may only get pictures.

ArcGIS viewers, Experience Builder apps, and ArcGIS Online web maps all sit on top of the same thing: the ArcGIS REST API. Once you find that endpoint, everything else is straightforward.

Step 1: find the REST endpoint

Open the map, press F12, go to Network, and reload. Filter for rest/services.

You are looking for a URL shaped like this:

https://services.arcgis.com/AbCdEf123/arcgis/rest/services/Parcels/FeatureServer/0/query?...

The important part is everything up to the layer number:

https://services.arcgis.com/AbCdEf123/arcgis/rest/services/Parcels/FeatureServer/0

That trailing 0 is the layer index. A service can hold many layers, numbered from zero.

Step 2: look at it in a browser

Paste that URL straight into a browser tab. ArcGIS REST endpoints are self-describing: you get a human-readable page listing the fields, the geometry type, the extent, the maximum record count, and the supported operations.

Add ?f=json for the machine-readable version.

Two things on that page matter enormously:

  • Max Record Count, usually 1000 or 2000. This is how many features the server will give you in one request. If the layer has 40,000 features, you will need to page through it.
  • Supported Operations. If Query is there, you can extract features. If it is not, the service is serving images only.

Drop the layer number to see the whole service and every layer in it. Drop back to /rest/services and some servers will show you the entire directory of everything they publish. That is often a far richer catalogue than the viewer exposed.

Step 3: pull the data

The query operation is the one that hands you features:

https://services.arcgis.com/AbCdEf123/arcgis/rest/services/Parcels/FeatureServer/0/query
  ?where=1=1&outFields=*&f=geojson

Piece by piece:

  • where=1=1 means "everything". It is SQL, so where=SUBURB='Thorndon' works too.
  • outFields=* means every attribute column. Naming the two you need is faster.
  • f=geojson asks for GeoJSON. f=json gives Esri JSON; f=html gives a readable page.

Add &resultRecordCount=1000&resultOffset=0 and increment the offset to page through a big layer. Add &returnCountOnly=true first to find out how many there are.

Add &geometry=<xmin>,<ymin>,<xmax>,<ymax>&geometryType=esriGeometryEnvelope&inSR=4326 to fetch only a bounding box, which is usually what you actually want.

Step 4: or skip it and use QGIS

QGIS speaks ArcGIS REST natively, so you often do not need any of the above. See how to open a FeatureServer in QGIS.

When you only get pictures

If the URL says MapServer and the service has no Query operation, you are looking at a service that renders images server-side. There are no features to extract, only a picture of them.

Sometimes a MapServer does support Query, in which case the same /query trick works. It is worth checking rather than assuming, and the difference between FeatureServer, MapServer, and ImageServer explains what you can expect from each.

Doing it the quick way

All of the above is finding a URL, cleaning it up, and remembering the query syntax. Geo Hound does that while you browse: it recognises every ArcGIS REST service type, saves each with its metadata already read from ?f=json, and gives you the URL formatted for QGIS or ArcGIS Pro with one click. It will also download a layer for an area you draw on a small map, which saves working out the envelope parameters by hand.

The usual caveats

The data is public because someone published it deliberately. It is not therefore unlicensed. Check the portal's terms, attribute the source, and do not page through a 40,000-feature layer once a minute on someone's server.