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.