CODEWITHSUNDEEP
sqlgisspatial
Colin DeClue
Colin DeClue

Reputation: 2244

Using SQL spacial data queries to calculate distance from two different projections

So, I've got some data that has longitude and latitude. I don't know what projection those are from. I've got some latitude and longitude I'll be fetching from Google maps API, which uses a projection with SRID of 3857.

If I just assume the data is from the same projection, and it turns out they're not, how far off could my distances be?

For instance, if they're from a 3-d projection (say 4326), but I just put them into a Geometry column with SRID 3857, and we're in the Northern Hemisphere, (Great Lakes area, but also other parts of the US), is there a way I can figure out how far off that would be?

Upvotes: 0

Views: 404

Answers (1)

lreeder
lreeder

Reputation: 12224

EPSG:3857 uses meters as units, while EPSG:4326 uses degrees. If you try to plot them on the same map without reprojecting one or the other, they will be very far off (many orders of magnitude) from each other.

You said you'll be fetching lat-lng from the Google Maps API, using a EPSG:3857 as a projection, but latitude and longitude coordinates are not projected by definition, although they may use a different datum. I can't find official Google documentation, but consensus seems to be that Google Maps API uses WGS84, same as EPSG:4326, so lat-lngs you pull from google maps API will probably fit exactly on top of others from EPSG:4326.

See http://spatialreference.org/ref/epsg/4326/ and http://spatialreference.org/ref/sr-org/7483/ and https://gis.stackexchange.com/questions/34276/whats-the-difference-between-epsg4326-and-epsg900913

Upvotes: 2

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