Re: Panning on azimuthal projections

Hi Karen,

As the projection author, that's up to you. For most of the projections
we have written, the origin lat/lon of the projection changes on a pan
and zoom, as you've noted. The screen x-y points that the forward and
inverse methods deal with are relative to the top left corner of the
map, so there is an offset (the pixel distances of height/2 and width/2)
subtracted from the original translation to help with that.

If you don't want the origin of your projection to change, you just have
to account for the new offset from the top left corner to the pixel
coordinate of the origin of your projection in the forward and inverse
methods.

The center of your projection should probably remain the coordinate of
the pixel in the middle of your map. But that doesn't have to be the
origin of your projection.

The MassStatePlane.java class in the unimplemented directory does this,
too, so you might want to check that out. It's pretty old, and suffers
from bit-rot, but it did the same thing you want to do.

Hope this helps,

Don

On Wednesday, January 23, 2002, at 04:37 PM, Karen Wei wrote:

> Hi,
>
> It seems that b/c azimuthal projections are based on a projection onto
> a plane
> tangent to the earth, there's no way of panning the map without
> resetting the
> center or point of tangency each time you change your view. I was
> wondering if
> there is any way to fix the point of tangency for an azimuthal
> projection and yet
> still allow panning capabilities other than zooming in and out. Any
> input or help
> would be greatly appreciated!
>
> thanks,
> Karen
>
>

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Donald Dietrick, dietrick@bbn.com
BBN Technologies, Cambridge, MA
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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