[OpenLayers-Users] Trying to draw a box on a map, and get the coordinates back.

[J. Cliff Dyer]

Thanks.  That's definitely a cleaner setup, but it didn't really solve
the problem I was having.  Before the box would look like it was being
drawn properly, but then the persistent copy would be vertically offset.
Now it's offset from the time I start drawing.  Which is an improvement,
because at least it will be easier for users to draw the box they want.
Here's a screenshot to show what I mean: 

http://imgur.com/5cJ4H

The Box was drawn exactly from Cape Fear to Cape Hatteras, marked with
little black dots on the screenshot.  However, the box itself starts
drawing a bit above the location of the cursor.  It seems the further I
get from the top of the display, the further displaced the box is.  It
is impossible to get closer than ~2cm from the bottom of the map.  There
also seems to be a very slight horizontal displacement.

Any thoughts on what might cause this displacement?  At this point, no
transformation is being done--at least not in any of my code.

Cheers,
Cliff



On Fri, 2009-11-13 at 09:58 -0700, Scott Lewis wrote:
> I did something like this using one of the examples on the OpenLayers 
> site, modified slightly.  Here's what I used:
> 
> boxControl = new OpenLayers.Control();
> OpenLayers.Util.extend(boxControl, {
>    draw: function() {
>      this.box = new OpenLayers.Handler.RegularPolygon(boxControl, 
> {"done": this.notice}, {sides:4, irregular:true, persist:true});
>      this.box.activate();
>    },
> 
>    notice: function(geom) {
>      // whatever you want it to do after the box has been drawn
>    }
> });
> 
> // then add the boxControl to the map
> 
> This works pretty nicely.  Note that "geom" will be in the same 
> projection as the map, so if you want the output to be in another 
> projection (like a latitude/longitude display) you will have to go 
> through the geometry and transform the points.  Also, you may have to go 
> through the geometry to determine which point is which (it doesn't 
> always start with the upper-left corner, and it's not always clockwise 
> or counter-clockwise).  If interested, I can post some of the functions 
> I do for that.
> 
> I hope this helps,
> 
> Scott Lewis
> NSIDC
> 
> J. Cliff Dyer wrote:
> > I'm trying to draw a box on a map, have it persist until another box
> > gets drawn, and submit the coordinates of the box to a form.  I'm doing
> > this to allow the user to search for objects within a given area.  
> > 
> > I'm doing this by using an OpenLayers.Handler.Box tool in a custom
> > OpenLayers.Control, which then uses to the "done" callback of the draw
> > method to create (or recreate) an OpenLayers.Feature.Vector and populate
> > (or repopulate) a search form.
> > 
> > My immediate problem is that the persistent box is not in quite the same
> > location as the box the user just drew.  I suspect this has something to
> > do with projections, but since I never explicitly set projections, I'm
> > not sure where the error sneaked in.  
> > 
> > More generally, I'm not sure I'm taking the optimal route from the
> > bounds passed to the callback to the Vector feature.  Currently, I do
> > the following:
> > 
> >   1) Take the incoming OpenLayers.Bounds object, 
> >   2) Get an OpenLayers.Pixel from the the corners
> >   3) Get the coordinates of each pixel using map.getLonLatFromPixel
> >   4) Create a new OpenLayers.Bounds 
> >   5) Extend it to my new coordinates
> >   6) Convert it to a geometry
> >   7) Convert it to an OpenLayers.Feature.Vector
> > 
> > Then I take the coordinates from step 3) and populate a form with them,
> > which I then submit to my application server side.
> > 
> > The code for this is pasted here: http://pastebin.com/m300c24a5
> > 
> > Any pointers on how to do this more cleanly would be much appreciated.
> > 
> > Cheers,
> > Cliff
> > 
> > 
> > 
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