Hi Diane,
It looks like those range rings in your images were created with radii
defined in decimal degrees/radians. They look the same, relative to
the graticule lines, in the two images. You'd get different results
for different latitudes if the range rings were drawn based on ground
distance.
The CADRG projection is a funny thing. The pixel/degree spacings
change depending on which zone you are in, and the projection tries to
figure out which zone settings are best for a given area. As the
scale changes, the zone might change, which leads to differences in
appearance.
Hope this helps,
Don
On Sep 23, 2008, at 1:12 PM, <dianefitch_at_comcast.net> wrote:
> Thanks,
>
> but I was wondering why it happens at only certain scales and
> doesn't matter where on the earth the circles are drawn.
>
> Diane
> ----- Original Message -----
> From: David Ward
> To: openmap-users_at_bbn.com
> Sent: Tuesday, September 09, 2008 11:39 AM
> Subject: Re: [OpenMap Users] CADRG progjection skews around scale
> 1:50000
>
> Diane,
>
> The "Squished/skewed" appearance you describe is an artifact of
> rendering the surface of a sphere (the Earth) onto a plane (the map)
> using different projection methods.
>
> Perform a search for "map projections" and you will find plenty of
> sites that will explain map projections.
>
> Cheers,
> David
>
>
>
>
> On Sep 9, 2008, at 8:06 AM, <dianefitch_at_comcast.net> <dianefitch_at_comcast.net
> > wrote:
>
>> Don,
>>
>> Has anyone encountered this problem.
>>
>> Using Openmap 4.6.3 or 4.6.4 application without any modifications,
>> when zooming in or out using CADRG projection, the image is skewed
>> around 1:5,149,249 scale and 1:7,000,000 once zoomed in closer tha
>> 1:5,000,000 the problem does not appear
>>
>> I drew a simple range ring about 1degree or lat and 1 degree of lon
>>
>> It is circular most of the time, but at certain scales it appears
>> "Squished"./skewed
>> The skewing seems to appear when the graticule lines form a square
>> (lat/longs equadistance??) Other scales the latitude
>> is wider than the longitude.
>>
>> This problem does not appear to happen in the Mercator Projection,
>> Orthogonal, Gnomonic
>> The LLXY projection the range rings are always skewed
>>
>>
>> Example pictures are around 38N Lat and 76W Lon
>>
>>
>> Thanks,
>>
>> Diane Fitch
>> Praxis Engineering
>> <skewedRangeRing.bmp><CorrectRangeRing.bmp>
>
>
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Don Dietrick, dietrick_at_bbn.com
BBN Technologies, Cambridge MA
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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Received on Tue Sep 23 2008 - 13:40:15 EDT