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radiantearth / stam-spec

SpatioTemporal Asset Metadata specification - defining core metadata fields for searching imagery & other geo assets
Apache License 2.0
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BBOX and footprints concerns (CRS, redundancy) #7

Open cholmes opened 7 years ago

cholmes commented 7 years ago

Right now we say that the BBOX is in CRS units, and we don't specify the units of the footprint.

At the very least we should specify the units of the footprint.

The example json is in 4326, but it is common for imagery to be provided in a specific UTM zone. If the bbox and coordinates are not in 4326 then it'll be pretty annoying for crawlers to properly catalog stuff, as they'll need to understand projections.

At Planet we have gotten a request from a provider to make the footprints available in the native projection. But it seems like most providers make the footprint available as 4326.

We could consider making 'projection' an optional field, and just rely on the image format to provide its proper projection? And then just make the BBOX and footprint in 4326.

Also do we really need bbox and footprint? Perhaps we just have footprint.

matthewhanson commented 7 years ago

@cholmes

An argument can be made to make all coordinates in 4326, it's easier to search by users and as you say doesn't require any understanding of projections. However I personally think that the footprint should be given in the projection that the data is made available in. Footprints in a data native projection will be rectangular (no doubt there are exceptions), while a 4326 footprint may be a more complicated polygon which may not necessarily describe the footprint as accurately as one in a data native projection. However, this is certainly going to vary by data source and provider. I think your idea of making projection an optional field is perhaps the best, it's not so good to require a footprint only in 4326 if it's not appropriate for some non-standard projections (see MODIS).

BBOX is a useful parameter - it's short and a quick way to use a spatial filter to narrow down a search region, as well as provide a quick reference for location. As such it works best in 4326, regardless of the footprint, and in fact is even more important if the footprint is not in 4326 as it provides easily recognizable coordinates.

In summary - BBOX is a short useful parameter when in 4326. Footprint should be allowed to be given in any arbitrary CRS but default to 4326 if a CRS is not given.