Re: Projection map ratio problem

Hi Raj,

But what you are seeing isn't be an OpenMap specific thing. With any
Mercator projection, the degree/pixel for the latitude direction varies,
decreasing as you go North, so a ratio between geo-coordinates
height/width will not be the same as a window height/width.

The thing that's hard in OpenMap is matching a projection from an
outside source with an OpenMap projection on the screen. Like Allan
said, OpenMap calculates the current projection with the center point on
the screen (which changes), while projected map data have a fixed center
projection reference point that may be offscreen. The LLXY projection
is essentially a reference free projection, though, which is why it
works for interacting with WMS servers.

And, this is why OpenMap can't draw pre-projected data residing in a
ShapeFile. There is a plan to fix this, I just need funding/support for it.

- Don

rs@syncline.com wrote:
>
> Yes! I reran the test with LLXY projection and the ratios
> stay the same. And I agree with the WMS client difficulty,
> which we've been having also.
>
> Allan Doyle wrote:
> >
> > Something to keep in mind is that the projections use the center of the screen as the
> > center latitude and center longitude. In other words, you're not sliding a fixed
> > projection around under a window, you're recomputing the projection for each pan/zoom.
> > That could be what you're seeing.
> >
> > It's precisely this behavior that makes it hard to use openmap as a WMS client or
> > server. The xxxview projections were an initial attempt at changing this.
> >
> > Allan
> >
> > rs@syncline.com wrote:
> >
> > > Mercator, by default for MapBean. Are you saying that the
> > > ratio of pixel and geographic rectangles should be
> > > different in Mercator, or are you saying that the
> > > variability in the ratios will change across the earth?
> > >
> > > Don Dietrick wrote:
> > > >
> > > > Hi Raj,
> > > >
> > > > What projection were you using to run these tests? I would expect them
> > > > to be different for different projections, and for different latitudes
> > > > within the same projection. This is affected by the degree/pixel ratio
> > > > characteristic of a projection, which is likely variable in most cases,
> > > > depending on latitude. Equal Arc projections (constant degree/pixel
> > > > ratios) should be OK, though, like the LLXY projection and the CADRG
> > > > projection within the same zone.
> > > >
> > > > - Don
> > > >
> > > > rs@syncline.com wrote:
> > > > >
> > > > > I've been doing some tests, and it seems that the ratio
> > > > > of the MapBean's screen width and height (r) is different than
> > > > > the ratio of the MapBean's geographic width and height (geor).
> > > > > See the results of a few extents below. Can anyone explain
> > > > > what may be happening here?
> > > > >
> > > > > minx -130.5 -84.2403 -85.2855
> > > > > maxx 131.0625 -75.2403 -74.1951
> > > > > miny -78.062 29.31935 28.64211
> > > > > maxy 78.06198 36.77261 37.37486
> > > > > w 465 465 573
> > > > > h 460 460 539
> > > > > gr 1.675351 1.207526 1.26997
> > > > > r 1.01087 1.01087 1.06308
> > > > >
> > > > > thanks,
> > > > > Raj
> > > > >
> > > > > --
> > > > > Raj Singh, Syncline
> > > > > rs@syncline.com
> > > > > +1(617)986-1000
> > > > > MapCiti.com...it's your town on the web
> > > > >
> > > > > --
> > > > > [To unsubscribe to this list send an email to "majdart@bbn.com"
> > > > > with the following text in the BODY of the message "unsubscribe openmap-users"]
> > >
> > > --
> > > Raj Singh, Syncline
> > > rs@syncline.com
> > > +1(617)986-1000
> > > MapCiti.com...it's your town on the web
> > >
> > > --
> > > [To unsubscribe to this list send an email to "majdart@bbn.com"
> > > with the following text in the BODY of the message "unsubscribe openmap-users"]
> >
> > --
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > Allan Doyle adoyle@intl-interfaces.com
> > International Interfaces +1 781 433 2695 (Office)
> > http://www.intl-interfaces.com +1 781 254 9484 (Mobile/Voicemail)
>
> --
> Raj Singh, Syncline
> rs@syncline.com
> +1(617)986-1000
> MapCiti.com...it's your town on the web

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