| Re: Map_Image and interpolation [message #18220 is a reply to message #18215] |
Thu, 09 December 1999 00:00   |
Ben Tupper
Messages: 186
Registered: August 1999
|
Senior Member |
|
|
In article <MPG.12b8d3f3d1de4d19899bb@news.frii.com>, davidf@dfanning.com
says...
>
> Well, I'm ready to concede the simple algorithm you
> found on my web page may not be what you want for an arc,
> but I think you might find an equivalent arc-interpolation
> scheme on Ray Sterner's web page. (That's where I found
> this algorithm.) Or, it would be easy enough, I think,
> to come up with one of your own. (Let's let the lurkers
> come up with something. :-)
>
Actually, I think Ray's algorithm should work fine... if I can get a handle on
how the warped image is georeferenced.
When I interpolate off the unwarped image I don't get the right answer because I
am assuming that the unwarped image pixel locations are all the same physical
size everywhere. But of course, they are not... because the east-west length of
an arc second decreases as you move poleward (lines of longitude converge.) So
if I try to calculate the distance between two points assuming a regular grid
cell size, then I get the wrong distance (and if I interpolate or sample the
grid at arbitrary locations between two points... I'll sample the wrong pixels.)
But you have suggested something nifty in the arc-interpolation idea. Hey, I've
got this computer and this software... maybe I could calculate distance in along
the surface of a sphere between two points. Ooo! Ooo! I like it!
Here's a thought; suppose I wanted to make the mapped image program interactive.
I want to post the lon/lat value of the cursor into two FSC_INPUTFIELD widgets.
How do I convert the cursor position into lon/lat using direct graphics?
>
> I always enjoy these conversations, Ben. I don't always
> understand them. But I do enjoy them. :-)
>
That's good... everybody needs a straight-man like me!
Thanks again,
Ben
Ben Tupper
PemaquidRiver@tidewater.net
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
