Re: [OpenMap Users] Openmap displays the VMAP coastline approximately 1.3 minutes of arc to the south of the correct position.

To: Tony Vasile <avasile@csc.com.au>
From: Don Dietrick <dfdietrick@gmail.com>
Date: 02/25/2012 02:41AM
Cc: openmap-users@bbn.com
Subject: Re: [OpenMap Users] Openmap displays the VMAP coastline approximately 1.3 minutes of arc to the south of the correct position.

Hi Tony,

The DTED layer in OpenMap doesn't show shoreline - the clear areas on
the layer are where the elevation is zero.  DTED data doesn't know
anything about shorelines.  The edge of the DTED display shouldn't
exactly match up with the edge of VMAP shoreline features, but I would
generally expect the DTED to be inside the shoreline features.  Is
that what you are seeing?

Depending on the level of VMAP data (Level 0 is based on 1:1,000,000
charts) and the level of DTED data (DTED level 0 has posts every 1km,
DTED level 1 has posts every 100m, or something like that), you won't
get them to line up perfectly - it's just a matter of the resolution
of the data and the fact that they aren't trying to show the same
thing.

For your map display, I would go with your VMAP data and use the DTED
to augment it by rendering the VMAP land areas with some transparency
so the elevation shading shows up behind it.

Hope this helps,

Don

On Thu, Feb 23, 2012 at 11:07 PM, Tony Vasile <avasile@csc.com.au> wrote:
> Openmap displays the tip of South Head of Sydney Harbour on the VMAP
> coastline at 33:51.734S 151:16.672E.
> On the same Openmap display, the position of the DTED data corresponding to
> the same point is 33:49.944S 151:16.876E (recognizing that the DTED data is
> more detailed and so the corresponding points are approximate).  This
> equates to a difference of approximately 1.8 minutes South. An independent
> check puts the equivalent DTED position at 33:50.102S 151:16.960E which
> confirms that the DTED position is correct. The VMAP display was then
> checked using VPFview2.1 which located the same point at 33:50.412S
> 151:16.665E. This corresponds  better with the DTED data and confirms that
> the source VMAP data is correct.
> This shows that Openmap is displaying the VMAP data with a displacement of
> 1.3 minutes South from the DTED data, which is a mix DTED Level 1 and Level
> 0.
>
> We are using OpenMap 4.6.5.
>
> Any suggestions on how to get the images to align better?
>
> Tony Vasile
> Senior Software Engineer
> CSC
>
> Level 3B, 26 Talavera Road Macquarie Park NSW 2113 Australia
> GBS | p: +61 2 9034 2404 | f: +61 2 9034 2398 | avasile@csc.com |
> www.csc.com
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