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seen Jul 25 '11 at 14:56

I'm a freelance software developer, specialising in custom geospatial software. If you're tackling difficult spatial problems, I'd love to hear from you: dan at shoutis.org!


Aug
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awarded  Yearling
May
29
awarded  Nice Answer
Dec
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awarded  Popular Question
Aug
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awarded  Yearling
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awarded  Nice Answer
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Jul
21
comment What kinds of line segments/edges require high accuracy in a true surface-of-the-ellipsoid representation?
Here goes, as concisely as I could make it. My reasoning is expressed in 3D Cartesian, not angular coords: (a) On a sphere, all points in a great circle are coplanar. (b) The transformation to the auxiliary sphere is linear and invertible. (Mistaken thinking?) (c) All points in an elliptical geodesic transform to points along a great circle on the aux. sphere. (d) All points on an elliptical geodesic are coplanar as well, due to (b). Finally, (e): Due to coplanarity, two candidate geodesic intersection points on the ellipsoid can be found by plane intersection.
Jul
21
comment What kinds of line segments/edges require high accuracy in a true surface-of-the-ellipsoid representation?
I'm quite sure I'm off track, but not sure where; I'd love it if you could help debug my reasoning. (Next comment.) Otherwise: thanks a heap for the helpful code + commentary + link; it's tremendously useful.
Jul
20
comment What kinds of line segments/edges require high accuracy in a true surface-of-the-ellipsoid representation?
A belated thanks for this reply. :) I'm too underwater to do more than skim at the moment, but it looks to be a a quite fantastic trove. A quick note on geodesic intersections since you called them out -- mostly for you to review as a check on my own undercaffinated intuition: Exact intersections of spherical geodesics can be found easily by intersecting the planes of the corresponding great circles, and that result carries over to ellipsoids by using an auxiliary sphere -- or am I missing something there?
Apr
14
comment what is the best way to programmatically convert between WKT and Proj4 string?
After some googling: spatialreference.org is powered by GDAL as well & uses the same code path (more or less), it seems.
Apr
13
comment Interpolate points between coordinates for smooth animation in google maps or openmap
@Kirk -- an library would make more sense than a web service. If smooth animation is your goal, you want to interpolate as quickly as the view can be updated. (And by no coincidence, libraries that support animation like jQuery already have interpolation & easing built in....)
Apr
12
comment Convert x, y position in georeferenced image (with world file) to longitude, latitude?
@antonj -- that's the EPSG code for the projection MerseyViking is using, which incidentally is not the same one as in your question. (Frequently used projections have a code assigned to them by the EPSG. I bet yours has an EPSG code too, but it wasn't included in the .prj).
Apr
12
comment Convert x, y position in georeferenced image (with world file) to longitude, latitude?
@antonj -- It's just the lazy (and web-linkable) way I used to convert your .prj parameters into proj4j arguments. The site gives you lots of ways to download a spatial reference, but the "proj4" link is the one you care about. They're all the same spatial reference info, just different formats. E.g. the "Lambert Azimuthal Equal-Area" in your .prj translates directly into the +proj=laea argument on the "proj4" version; the "central-meridian" of 20 translates to +lon_0=20, etc.
Apr
12
comment Convert x, y position in georeferenced image (with world file) to longitude, latitude?
Upon a closer read, I'm not certain your +0.5 is correct. (See the comment I left in the discussion on the main question.)
Apr
12
comment Convert x, y position in georeferenced image (with world file) to longitude, latitude?
@whuber @antonj -- my prior comment is half-remembered, but confirmed by the wikipedia link I gave in my answer. I wanted to add anyway that I'm fairly sure about the tfw and pixel centers but not 100% certain.
Apr
12
comment Convert x, y position in georeferenced image (with world file) to longitude, latitude?
@whuber @antonj -- just a note that the .tfw already includes an offset to the pixel center; adding another 1/2 will put you at the bottom right of the pixel. (Line 5 & 6, the "origin" of the affine transform in the .tfw, are the coordinates of the center of the top-left pixel, not the coordinates of the top-left of the top-left pixel.)
Apr
12
answered Interpolate points between coordinates for smooth animation in google maps or openmap
Apr
12
comment Convert x, y position in georeferenced image (with world file) to longitude, latitude?
Amusing... we gave, more or less, the exact same answer at the exact same time. ;)