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Sparse Datasets and empty tiles

TLDR

Basemaps should continue to serve transparent images for areas which do not have any data, for maximum compatibility with clients. Once clients upgrade and fix bugs, either HTTP 204 No Content or HTTP 200 Ok with a 0 byte payload responses would be preferable.

Further investigation should also be done on OATiles-hint: empty response headers, as well as converting more clients to TileJSON which allows specify a bounding box to restrict where a client requests tiles from.

Background

LINZ has imagery datasets which are very sparse, with large empty areas between smaller sections of valid data, for example Gisborne Urban 2022-2023.

Gisborne 2023

Even when providing a bounding box for the imagery, there is still a very large amount of empty space, which results in a large number of empty tiles being served.

Question

What is the best way to tell clients that the area they are requesting has no data?

Response Options

A server can return different responses to clients to signal that there is no data. The following section is specifically for WebP, JPEG and PNG imagery tiles returned from a tile server.

For Mapbox Vector Tiles, a 0 byte response of either 204 No Content or 200 Ok works in clients as a 0 byte vector tile is a valid vector tile.

200 Transparent Image

Serve a valid image in the request format (e.g. WebP, JPEG, or PNG). For example if a client requests a PNG, serve a fully transparent PNG tile.

Pros:

  • ✔️ Supported everywhere

Cons:

  • ❌ JPEG does not support transparency, so users get a black image
  • ❌ Much larger responses: 1KB vs 0 byte payload
  • ❌ Difficult to know in logs if the tile request was for a valid region

200 - 0 Byte

Serve HTTP 200 Ok, With a content-length: 0 response header, which is a 0 byte response

Pros:

  • ✔️ Supported in ArcGIS Pro and QGIS
  • ✔️ Supported by ArcGIS Online, Leaflet, and OpenLayers
  • ✔️ Supported by maplibre since v4.0.0
  • ✔️ Supported by HTTP RFC, which allows 0 byte 200 Ok responses as long as the message framing indicates this (via Content-length=0 response header
  • ✔️ Recommended option from OGC Tile API

Cons:

  • ❌ Not supported by mapbox gl
  • ❌ HTTP RFC suggests that while valid, servers "ought to return 204 No content"

204 No Content

Return HTTP 204 No Content, with no body

Pros:

  • ✔️ Supported in ArcGIS Pro and QGIS
  • ✔️ Supported by Leaflet and OpenLayers
  • ✔️ Supported by maplibre since v4.0.0
  • ✔️ Supported by HTTP RFC: "server ought to return 204 No content"
  • ✔️ Recommended option from OGC Tile API

Cons:

  • ❌ Not supported by ArcGIS Online
  • ❌ Not supported by mapbox gl
  • Not always cached by browser

404 Tile Not found

Return a error state saying the tile is not found

  • ❌ Is an error state
  • ❌ Not always cached: browsers / CDNs often do not cache 404 responses

References

Simple proxy that converts 204 No Content into different results.