# Translate coordinates between different CRS

There are two Coordinate Reference System, say they are wgs84 and crs2(a local crs).

A certain location can be translated from wgs84 to crs2 by official algorithm, and the algorithm is irreversible(Because the algorithm is confidential).

Now we have a lot of locations in crs2, and we want to translate them to wgs84.

We tried to use the grid to make this.

For example, we can generate millions of points in wgs84 which have a certain distance with each other, then they will make up a gird.

Then we translate these points to crs2 one by one by the algorithm. After that, for a given point in crs2, we first find the gird it reside in , and then calculate a approximate location in wgs84.

Like this: And the precise depend on the distance of the gird, the small the better.

I am just not sure if there is any models or best practices out of box for building the grid and make the reverse calculation as fast as possible?

• might help us to know what cs1 & cs2 are – Ian Turton Apr 1 '15 at 11:00
• They are all local crs, seriously they can not be called as crs. – giser Apr 1 '15 at 11:05
• "There are two Coordinate Reference System" versus "seriously they can not be called as crs." How is the algorithm irreversible? Are you talking about a resampling problem? – Martin F Apr 1 '15 at 14:18
• I just though that you can take the crs2 as the china mars coordinate system, and crs1 as normal wgs84. – giser Apr 1 '15 at 14:45
• This is basically the way that datum shifts with ntv2 grids work (in both directions with the same grid file), but they are developed to exchange between degree coordinates. – AndreJ Apr 2 '15 at 3:55

• Ok, I got it, thank you. And in fact, why we use the grid calculation is that the crs2 is complete locally and classified by a third organization, so we do not know if the transformation is Equation based or not. If not, does the modeling solution still work if I choose the parameter as complex as possible? In this case(the wgs84-crs2 is not equation based), it seems that this becomes to a complete mathematics problem: find a polynomials `f` which meets:crs2xy = f(wgs84xy), then solve the `f`? – giser Apr 9 '15 at 13:23