Question regarding coordinate transformation

Hello,

after searching around for a couple of hours I kinldy ask for help in
this newsgroup. Here is my task: I want to use GoogleMaps and add
Polygons to the map.
I've setup a test page with a little Javascript. After the map is
moved by the user, the Javascript sends a request via AJAX to a
backend application I wrote. It sends the coordinates as delivered by
the maps getBounds() funnction, which are in latitude and longitude.
The backend is developed in Delphi. I use the ShapeAPI to read the
contents of a Shapefile. I would like to find out which shapes lie
within the "viewport" (the currently visible area in the Google map).

Therefore I have to map the coordinates somehow. Problem is that the
values in the shapefile are in a different ccordinate system. I spent
some time finding information, but with a possible solutions I found,
I get to a point where I don't know how to proceed.

The shapefile (consisiting of several files) inlcudes a file with the
ending "prj" which contains information regarding the coordinate
system, projection, datum etc. How can I use that to either transform
those shapefiles coordinates to lat/lng values or to coordinate the
lat/lng values delivered from Google to the shape files coordinate
system?

I tried to use http://www.windreader.com/geodesy. It has a function
called LL2UTM. But as far as I can see my shapefile is - according to
the info in the prj file - transverse mercator, and not "universal
transverse mercator", which I thought is something different (by the
way, how would zone numbers and zone letters be stored in a
shapefile?) Anyway, that function needs a "geoid" besindes the lat
and lng values, and I don't know where to get this geoid from.

Even when I tried to do some transformation even not in Delphi, but on
soem webpages, I always end up at a point where I don't know how to
set certain parameters.

Example:
- The datum tag in the prj of one of the shapefiles I'd like to use
file says "D_Deutsches_Hauptdreiecksnetz". WIthin there is another tag
called spheroid, that contains"["Bessel_1841",
6377397.155,299.1528128]"
- When I go to http://www-ipf.bau-verm.uni-karlsruhe.de/cgi-bin/trafo.tcl
I am asked for the in and out coordinate system and even in the list
of predefined systems there are three options containing "Bessel". So
which one to take?

What I would like would be a solution where I could "just" enter the
numeric values given in the prj file of the shape file somehow and
then enter a point coordinate of the shape file data and get its lat
and lng values.

Immposible? I cannot believe that. I know that this is not trivial and
that some deep insight in the whole matter is needed, but I am truly
willing to learn. ;)

Any help will be very appreciated.

Best regards,

Christian Kirchhoff
.

Relevant Pages

  • Re: Open Source Web GIS Tools
    ... It handles reprojection, shapefiles, database sources, and all sorts of things. ... You'll need some data to make the map out of, and to style it to your liking. ... Google could've helped you, but like you said, if there's no data for the country in question ... ... using a shapefile and latitude/longitude positions. ...
    (comp.infosystems.gis)
  • Re: Land or Sea???
    ... Any GIS will do this. ... Google for world data shapefile. ... Find and download a shapefile for Denmark. ...
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  • Re: Putting dots on a map
    ... If you want to wait until you get home I use a large laminated wall map ... the same free license as theirs. ... If you want to be precise about it, go get a copy of QGIS (http://www.qgis.org ... shapefile and add points to your heart's content, ...
    (misc.transport.road)
  • Re: Putting dots on a map
    ... If you want to wait until you get home I use a large laminated wall map ... the same free license as theirs. ... If you want to be precise about it, go get a copy of QGIS (http://www.qgis.org ... shapefile and add points to your heart's content, ...
    (misc.transport.road)
  • Re: a shapewrite question
    ... >> I'm using to eventually read the shapefile is complaining about the ... >>> Matlab's shapefile tools ignore the projection component of a shapefile, ... > i.e. x/y coordinates based on a projection rather than straight lat/lon? ... This is what's listed in the .prj file of the few non-projected, ArcGIS-generated shapefiles I have floating around my computer. ...
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