What are the consequences of using other geojson projections while creating vector tiles using tippecannoe?

Question

I have written my own vector tiles service based on vt-pbf, which queries a really large source of geojsons which specify different kinds of projections to be used.

While using tippecanoe tool I am getting warnings like

 Warning: GeoJSON specified projection "urn:ogc:def:crs:EPSG::26915", not the expected "urn:ogc:def:crs:OGC:1.3:CRS84"

 If "urn:ogc:def:crs:OGC:1.3:CRS84" is not the expected projection, use -s to specify the right one

Can someone explain/direct me towards the basic differences between various kinds of projections, and also the consequences I will be facing after generating these tiles with these warnings?

Answer 1

According to the vector tile spec:

Vector Tiles MAY be used to represent data with any projection and tile extent scheme.

So if you really wanted, you could store data in another projection, as long as whatever was reading it knew what was going on. You wouldn't be able to use Mapbox-GL-JS (I think) because it only supports EPSG:4326 (latitudes and longitudes).

But it doesn't sound like you want that.

Can someone explain/direct me towards the basic differences between various kinds of projections,

In a nutshell, instead of latitudes and longitudes stored in the GeoJSON (eg, coordinates: [145, -38]), other projections define different coordinate systems, usually based on X and Y positions on some flat map. For instance, that same point might be [19384792, -852341]. Only latitudes and longitudes are allowed under the current GeoJSON spec though.

Most likely what you want to do is reproject the GeoJSON during the conversion, as suggested:

tippecanoe -s EPSG:26915 ...