Re: [OpenMap Users] CADRG image doesn't always render fully

From: Don Dietrick <dietrick_at_bbn.com>
Date: Thu, 27 Mar 2008 11:11:42 -0400

Hi Ashley,

What you are seeing is an effect of the layer trying to use the right
data for the projection. The projection that CADRG relies on is
divided up into 18 zones, 9 in each hemisphere. The first zone is
from 0 degrees latitude to 32 deg lat, the second is 32 to 48, the
third is 48 to 56, etc. Each zone has different pixel spacings for
different chart types, and if you look at the data files for CADRG,
you can see that the image files are organized by zones (the zone
number is the last character in the file extension).

The OpenMap projection decides what zone should be displayed based on
the latitudes being displayed on the map. The RpfLayer uses the data
for the current projection's zone.

RPF data wasn't designed to be used when the map is really zoomed
out. For a given chart, the coverage for a zone does have overlap
with the zone above/below it, with the idea being that as you pan up
or down, you can get to a point where you change which zone coverage
you use. When you zoom out, the limitations of the zone overlaps are
exposed, and that's what you are seeing. In the first image, the
projection is deciding that the upper zone should be displayed, so the
layer is showing that data. For the second image a different zone is
being used.

In order to get the effect you want, the RpfLayer needs to be modified
so it doesn't assume a single zone for the map. It already does that
to handle the equator, but that's a special case. The modifications
would have to extent that case to check for all the possible zones on
the map, and fetch coverage as a result of those determinations. I
have these modifications planned, but not scheduled, as they are
unfunded, so right now they will be implemented when I get a chance to
get to in sometime this year. If that might be something that you
would be interested in sponsoring, please let me know and we can move
it up in the queue.

Hope this helps,

Don


On Mar 26, 2008, at 8:41 PM, ashley_c_mort wrote:
> We are seeing a problem where our GNC CADRG layer will not always
> fully render (see image)
> http://www.mortworld.com/OpenMapproblem.gif
>
> If I zoom in just a bit, then the GNC layer will render even in the
> areas that were in the "cut-off" area in the first link.
> http://www.mortworld.com/OpenMapSlightzoomin.gif
>
> Any ideas on what could be causing the GNC image to only display
> half way in the first link? We are using OpenMap 4.6.3 and tried
> 4.6.4 but saw the same issue. Any ideas are greatly appreciated!

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Don Dietrick, dietrick_at_bbn.com
BBN Technologies, Cambridge MA
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=



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Received on Thu Mar 27 2008 - 11:14:59 EDT

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