Re: [OpenMap Users] General map questions: Shifting points ... strange

From: Don Dietrick <>
Date: Tue, 20 Jul 2004 09:10:40 -0700

Hi Karl,

On Jul 19, 2004, at 3:46 AM, Karl Koch wrote:

> Hello everybody,
> I have some questions about a more general topic not particularily
> about
> OpenMap.
> Background: I have a picture of a map which I have with some effort
> geocoded
> using reference points. I have some locations to map to this map. I
> geocoded
> those location using a internet mapping service which provides WGS84
> locations. I used the same service for the reference points for
> putting the
> map at hte place.
> Now, I added the first location point provided as longitude latitude in
> WGS84 to my map and it was fine. However when I added more points I
> realised
> that there were shifted somehow. Now my questions (Qx):
> Q1: How can the points be shifted, even it I use the same mapping
> service
> for putting the map picture in place and geocoding the location
> points. It
> is both the same coordinate system (WGS 1984).

I'd look at how they are shifted, and look for trends. Are they all
off by the same pixel amount, regardless of scale? Does the shift
amount depend on lat/lon?

> Q2: In general, what are reasons for that effect that points shift even
> though the same system is used, BUT different maps.

I'd also check the source of the data, and make sure the datums for the
sources are truly the same. Bad input == bad output.

> Q3: Does anybody know how this problem is officially called and where
> I can
> specifically read more about it (want to write about it and need
> therefore
> more information).

Datum mis-match?

> Q4: Does somebody know more general mailing lists where I can ask
> questions
> about mapping in general (geocoding, mapping, technologies related
> with the
> mapping business etc.) ?


- Don

Don Dietrick,
BBN Technologies, Cambridge, MA

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Received on Tue Jul 20 2004 - 12:12:25 EDT

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