Re: [OpenMap Users] Mercator x.y

From: Don Dietrick <>
Date: Tue, 3 Apr 2007 14:32:04 -0400

Hi Charles,

For OpenMap projections, the center point is the center of the map
(window), or just some basic reference into the 2D x, y space. The
height and width of the projection obviously represent the height and
width of the map window, but there's a subtile importance in their
values because the x, y locations are based on the origin in the
upper right corner of the map window, where x values are positive to
the right and y values are positive going down (this matches the Java
rendering coordinate system).

If you are using OpenMap projections for conversions without
visualization, the center, height and width values become more
arbitrary, but you'll probably have to do the math to get the values
in the 2D x, y space you want (like flipping the y axis, and/or
adding width/2 to x values, etc). For Mercator, the actual values
you choose are probably not that important, and you can reuse the
same projection object to do all of the conversions. You might want
to create a Mercator projection at (0, 0) lat/lon with a scale value
you like, and find out where (90, -180) and (-90, 180) are in x, y
space, and use that for your height and width for a different
Mercator object that you'll actually use.

Hope this helps,


On Apr 3, 2007, at 1:52 PM, Hodanics, Charles J. wrote:

> Hello OpenMap community,
> I am new to mapping and attempting to use your Mercator class to
> perform x,y <> lat long. I am curious if there is an easy way to
> create this mapping by providing known (x, y) (lat, long) pairs. I
> am using some poor mapping utilities that do not provide the center
> easily. The maps may also be in flux and given known lat,longs I
> can easily find their new x,ys.
> In short I would like to diver from having a center lat/long and
> width height constructor. Is there an easy way to do this?
> Thanks,
> Charles Hodanics
> 443-778-3246

Don Dietrick,
BBN Technologies, Cambridge, MA

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Received on Tue Apr 03 2007 - 14:32:22 EDT

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