Grasshopper

algorithmic modeling for Rhino

There is a big problem with the Topo Points/Curves/Surface generator: it does not factor in the shape of the earth when producing the surface.  Let's say the longitudinal domain is set as -2:-1 (2 degrees west to 1 degree west).  Changing the latitudinal domain from, say, 0:1 (the equator to 1 degree north) to 88:89 (88 degrees north to 89 degrees north), has zero effect on the x,y shape of the topography map generated.  However, in reality, the map should be far, far thinner in the latter case, because longitudinal lines get closer together toward the north and south poles.  In actuality, the shape should be close to a trapezoid in both cases, but this is probably not a necessary detail for most people producing maps, since, at an urban or smaller scale, the latitudinal lines bounding the north and south of the map will probably not be that significantly different in length.  But the maps should at least stretch from close-to-square for a 1 degree x 1 degree map near the equator to an extremely thin rectangle for a 1 degree x 1 degree map near the north pole.

As an example, I'm looking at a location in Sheffield, UK.  The relevant SRTM HGT file spans from 53 N to 54 N, and 2 W to 1 W.  The length of the map in the north-south direction should be approximately 111 km, as is the case with the topo map generated by Elk (and a near-standard for 1 degree latitude anywhere in the world).  The length of the map in the east-west direction, however, should be somewhere in the range of 67 km, since the 2 W and 1 W longitudinal lines are much closer together at this latitude than they are at the equator.  Thus the map should be nearly twice as long in the North-South direction as it is wide in the East-West direction.

If this were to be sorted out, I think it would be really nice to then have the SRTM topo map be positioned automatically in relation to the OSM map being brought in.  I think it's good that the OSM map is positioned at 0,0, rather than it's world coordinates, but maybe the SRTM topo map could be aligned with it based on the latitude and longitude domains we input to the SRTM grasshopper module.

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Replies to This Discussion

What you're describing is an inherent problem when translating data from a spherical grid to a cartesian one.  

One of the tricks involved with "sorting this out" is that achieve absolute accuracy, as your post requests, the osm points would also have to be curved in the Z axis as well.  To use these points to then produce accurate geometry, you would be modeling on a curved workplane.  

Have fun with that. 

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