[OpenLayers-Users] Orthorectifying

[Christopher Schmidt]

On Sun, Sep 30, 2007 at 01:30:14PM -0600, Linda Rawson wrote:
> I think I can get this one but if someone has a formula to help me that
> would be great.
> 
> Basically he uses the panning tool.  The north, south, east, west tool.  So
> when he goes east he does not want to evenly go to the next square.  He
> wants to go to the next square based on a sphere.  If he was on the equator
> then the pixels would be correct.  But if he is in say Canada he wants to
> panning tool to go to the next square in the sphere.  I hope this is making
> sense.

Okay, I think I get it.

The world is a kinda-spheroid like thing. The default OpenLayers
projection is a lon/lot geographic coordinate system -- this means that
50 pixels north is 50 * map.resolution degrees north. However, what
number of *meters* that is on the ground is variable, based on where the
lat/lon are.

So, in order to make the spheroid world a 2D plane, we 'project' it. The
lon/lat geographic projection is just one option here -- others are, for
example, mercator, or peters equal area, etc.

It sounds like the person you are talking to wants a projected map.
Depending on the type of things this person cares about, this may be
easy or hard. 

http://openlayers.org/dev/examples/spherical-mercator.html is a
set of projected maps. You can see that the WMS service is providing
data not according to lat/lon, but according to projected units. To the
user, it doesn't really make any difference, if they don't see the units
-- they just know that they go up, and as they go up, the amount of
latitude/longitude they go up by every time they hit the button is
different, instead of the same.

It's not really clear to me who 'the map providers' are in this case.
Does he or she mean Google? If so, then the sphericalMercator support in
2.5 is probably just the kind of thing he is looking for, as it will let
you lay images on top of these and tile them properly. If the customer
is talking about some other data provider... well, it's hard to know
what exactly he (or she) means.  

Perhaps with a bit more information about the scenario you are exploring
would be helpful here. 

> I create the image based on the lat/lon coordinates passed to me with open
> layers.  The mapping service I use just needs the upper left and lower right
> corners to return the images.

Are you just using one layer? Where is the image coming from? Do you
have a commercial base layer? something else?

Regards,
-- 
Christopher Schmidt
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