Hi Julian,
On Feb 8, 2005, at 10:04 PM, Twohig, Julian wrote:
> Hello,
>
> I am writing a customised GIS back-end in Java for a mission planning
> system.
>
> I am currently implementing the VPFLayer of Openmap for requesting
> VMap data.
> As an overview of what I am trying to achieve, Openmap always requests
> Vmap data for a single visible region within the GUI. My application
> splits the world up into tiles and each of those tiles requests data
> once they are visible. I want to be able to control the number of
> tiles high and wide to break the world up into, and when each tile
> requests Vmap data through Openmap only points for that region should
> be returned at a detail commensurate with my current scale.
>
> I have come across a few issues with this as follows:
>
> 1) I only want points for a specified region equating to the size of
> our display tiles but Openmap returns all the points for a tile in the
> VMap database. For example, if I request an area that is 2 degrees lon
> by 2 degrees lat I get back point coverage for a 15 degree by 15
> degree area for the Vmap tile that my requested region intersected.
> Is there any way within Openmap to specifically limit the retrieved
> points to that area defined by the LatLonPoint(s) upperleft and
> lowerright?
You can look at the VPFLayer.prepare() method to see how the
LibrarySelectionTable is used with the Warehouse to retrieve
OMGraphics. You can make any query to the LibrarySelectionTable you'd
like.
> 2) Can you reduce the complexity of the returned OMGraphicList from
> VPFLayer.prepare? I would like less points returned the further out I
> zoom to enable fast zooming and panning. At the moment regardless of
> my zoom and the scale, width & height that I send through I always
> receive full point complexity.
Please feel free to modify the code to create a warehouse that creates
OMGraphics the way you like, thinning the data points used to create
the various OMGraphics. You should be aware that the number of points
read out of the database can't be adjusted, so you'll be still be
putting up with the work reading in all the points for an area. - I
don't know if the work performed to thin the data on the fly would be
worth the time saved in rendering fewer points, but you could try it.
Alternatively, you could use the ScaleFilterLayer to alternate between
using something like shape files for zoomed out scales, and switch to
VPF when you zoom in.
Regards,
- Don
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Don Dietrick, dietrick_at_bbn.com
BBN Technologies, Cambridge, MA
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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Received on Sun Feb 13 2005 - 19:08:21 EST