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Overlaying Maps: 1

As said in the previous chapter, more than one window can be shown in the browser, each with its own map.
. As you can see, the Stalpaert map has been rotated: the original had the south at the top. This map was done by the georeferencing routines of
which are usually applied to satellite imagery, but work of course just as well for scanned maps. It took about one and a half hour to reference this 400M scan.
Both maps can be zoomed and panned independently
, but as they now have the same coordinate system, this can also be done simultaneously for both
. e.g. for zooming in to the Westerkerk
. You can see immediately that the pattern of streets and canals looks very much alike in both years. However, we would like to be able to superimpose both maps, to see how exact the matching is. Superimposing two images is no problem at all in modern browsers
, and both maps can be turned off individually for comparison
. Zooming in and out leaves the relationship between both maps unchanged
.
An advanced feature of MapClient is that each map is generated on a different WebServer. The browser sends requests for maps with the same extent and size to the servers and waits for the indivdual maps to return. Only then overlaying is done; as the maps have been computed to fit exactly, this is just a matter of superimposing the two returned images. I did this on the
of the University of Amsterdam, but it can be done just as well on any pair of computers connected to the Internet. More generally, any map can be produced from layers anywhere on the Internet. This is the same concept as the
is working on. Currently the OpenGIS standards for WebMapping (
and
) are still somewhat restricted. MapServer already supports them, but above that the MapClient interface makes available the full power of native MapServer for creating distributed maps on the Internet. Note also that a third OpenGIS standard, for
based geographical features is also supported by MapServer via the
link. Finally, the MapServer's complete functionality is programmable at the server side in the Perl, Python and TCL languages. All this functionality can be accessed from the browser by MapClient.