WMS services

A WMS is not a file: it is a service that draws pictures on demand. So the query hands over an address and a name rather than data, and the panel builds the dataset and its layer.

SELECT 'https://services.geoglows.org/geoserver/satellite_based_precipitation/wms' AS wms_url,
       'persiann_pdir_24h' AS wms_layer;

Role	Detected from
WMS URL	wms_url, wms, service_url
WMS layer	wms_layer, layer, layer_name

The two go together: a service publishes many layers, so a URL without a layer name is not a picture. Both must be present or neither role applies. Like the raster role, neither appears in the Field mapping editor — alias them in the query if your columns are named otherwise.

Nothing sits in between

There is no tile server to configure here. That is the difference between a WMS and a COG, and the reason this works on a Grafana with no infrastructure behind it at all: a COG needs a server because a GeoTIFF is not tiles, whereas a WMS is the server. The Raster tile server panel option plays no part.

What the query produces is a dataset with no rows — grafana-<refId>-wms — holding where the service is and which of its layers to draw, alongside the ordinary table dataset your rows make.

Give it the bare endpoint

https://services.geoglows.org/geoserver/satellite_based_precipitation/wms, with no query string.

A whole GetMap URL pasted into the column is trimmed back to that, deliberately. Every request is built by appending parameters to what you give, and the appending does not check whether a ? is already there — so anything after one survives only as a corrupted first parameter, and the server answers with a parse error about a date it was never sent. Since pasting a whole GetMap URL is the most natural thing to put in a column, the choice was between reading it charitably and drawing nothing.

The cost of that is a service which genuinely needs a parameter on its endpoint — a MapServer ?map=/path.map. Those cannot be driven this way at all.

The clock

Many environmental services publish one image per day or per hour and expect a TIME on the request. The map's time widget walks those dates, and the dates can come from the query or from the service itself. That is the subject of its own page: Imagery over time.

Worth knowing here: kepler draws a WMS but has no notion of its time dimension, so a time-aware service would answer with its default slice for ever. Putting the date on the request is the panel's doing.

Clicking asks the service

A WMS carries no rows, so the only way to know what is under a pixel is to ask the server — GetFeatureInfo, which is what a click does on a queryable layer. kepler shows the answer in its popover, and a click mapping on the field wms_value sends it to a dashboard variable, so the panels around the map can read it. See Cross-filtering.

wms_value is the first value the service returned, under a name that is the same everywhere. The attributes also arrive under their own names — both as the service spells them (GRAY_INDEX) and as kepler displays them (GRAY INDEX) — for a vector WMS returning several.

The question carries the date on screen, so the answer is about the image you are looking at rather than the service's default. And it is the service's answer verbatim: a sentinel like -99 means no data here, not a failure.

A click that seems to do nothing

kepler's time widget sits over the bottom-centre of the map and takes the clicks that land on it. That is indistinguishable from a layer that cannot be queried. Click somewhere else on the map before concluding the layer is not queryable.

What to expect of the service

• CORS. The browser fetches the pictures, so the service has to allow it. This is not the COG arrangement, where a server in the middle does the reading.
• A strict CSP needs the host in connect-src, not img-src. A WMS image is fetched and parsed as data rather than pointed at by an <img>, so the obvious directive is the wrong one — and the symptom is a map that draws nothing, silently, with no CSP message about images. See Install.
• The capabilities document is read once per service per page load, and it can be large: nearly a megabyte on GeoGLOWS, whose hourly layers list fourteen thousand dates.
• A date the service does not have comes back as an empty picture. That is why nothing is ever guessed: with no calendar from either side, no TIME is sent and the service answers with its own default.

Reading it instead of drawing it

A WMS hands out pictures, which is a wall for anything you want to compute. But a GeoServer that publishes a WMS usually publishes a WCS over the same coverage — the same scene, delivered as pixels rather than as a rendering — and that turns the imagery on the map into something a query can work on, with no second source and no ETL.

See Measuring imagery.