I was measuring geometry using some PostGIS functions and noticed that I was getting different results from the two methods I used.
sum(st_length(st_transform(geom, 3857))) # Gave 3674 sum(st_length(geom::geography)) # Gave 1671
Why am I seeing different results? I was expecting a slight deviation but this seems to be quite significant.
Is there something wrong with how am I using these queries?