Re: help!

On Jan 26, 6:33 pm, "Bartc" <b...@xxxxxxxxxx> wrote:
<DarkAngel...@xxxxxxxxx> wrote in message

news:25bcd50c-45c4-4480-aaa8-944c1beab834@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

ok, i need help figuring out this homework problem from Into to
Programming. I've got to find the lat/lon coordinates of a point.
Here's what I know:
pixel coordinates of the point (382,387)
the lower left corner of image is (0,0)
the pixel dimensions of the image (512,512)
lat/lon coordinates of the upper left corner of image
(36.261992,-89.857178)
lower right corner (36.244273,-89.835205)

I know this is basically a conversion problem, but how?

This sounds like basic arithmetic unless there's some catch (non-linear
lat/long for example).

The lat/long corresponding to (382,387) in your map of (512,512) would be:

lat = (382.0/512.0) * (36.244273-36.261992) + 36.261992
long = (387.0/512.0) * (-89.857178-(-89.835205)) + (-89.835205)

This assumes, if the image is 512x512 pixels, (0,0) and (382,387) are the
points at the bottom left of each pixel, while the top right point is
(511,511) (or (512,512) for the top/right of that pixel). But your
assignment may have different assumptions.

There are a couple of projections for which lat/lon are linear on a
map, but you should not count on it.
You need to know the projection, the reference ellipsoid and you need
to have the inverse projection.

Proj.4 is a package used to deal with this sort of thing.
http://proj.maptools.org/

Of course, it is also possible that the teacher knows nothing about
cartography and thinks you can use linear interpolation to get the
answer.
.

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