There are hundreds or thousands of maps on the web which are not
georeferenced. The
content included in these maps — be they historic, hand drawn, or maps
which offer more specific detail of a given area like a National Park —
can be made more interesting by coordinating their display with other
maps of the same area. Currently, the tools to convert images tend to be
geared towards GIS professionals — georeferencing an image is a chore,
something that could be improved upon with a new interface.
In an effort to help non-GIS professionals associate their images with
places in the real world, we have developed a
href="http://labs.metacarta.com/rectifier/">web-based map rectification
tool, which is available now for public use. The service will allow
you to upload an image, and using a reference map, you can select ground
control points — points from the reference map which match up to the
uploaded image — and then warp the image. You can use any of a number
of default base maps to reference against, or add your own WMS or
KaMap layers to the map and use them to find ground control points.
Warping the image creates two kinds of output. The first is a GeoTIFF, which can be
downloaded for use in desktop GIS. The second is a WMS URL, which can be used for copy-and-pasting into OpenLayers applications.
The service also includes the ability to comment on maps — so if you
see something that someone uploaded that you really like, you can let
them know.
I’ve spent all weekend playing with this — georectifying images from
maps from National Parks to
href="http://labs.metacarta.com/rectifier/map/27">Chernobyl Radiation hot
spots. It’s really
fun and interesting to see informative maps move into the real world
space — to see just how
href="http://labs.metacarta.com/rectifier/map/15">widely the Persian
Empire spread, or to see a handy
href="http://labs.metacarta.com/rectifier/map/25">Map of Isle
Royale.
Try it out. Load up a map — you might have one lying around, or you
might look through the
href="http://www.lib.utexas.edu/maps/">University of Texas Map
Library — give it a title and description, and then add GCPs.
First, double click on the left hand map, on some feature you can pick
out on a map. Then move to that feature on the right, and double click
– you’ll see the lat/lon of the marker on the left change. Once you’ve
got 4 or more points, hit the warp button in the page, and watch as the
map on the left appears on the right.
If you’ve got a map with town names on it, instead of double clicking on
the town on the reference map, you can use the MetaCarta Labs GeoParser API to
look up the location. Simply enter the location name in the box on the
lower right, and a marker will be added to the reference map. While a
GCP is open, click on this newly created marker to adjust the location
of that GCP.
Personally, I think this is the coolest tool I’ve seen written yet with
OpenLayers. I’d also like to throw in a line of thanks to all the
software it’s built on: GDAL is used to do all the image warping, and the
GDAL community was very helpful in creating the app, Cheetah and sqlobject
help make web development easy and even fun, and Mapserver serves maps, as
it always does, like a champ.
We’re looking for feedback as to what ideas you have for what you can
do with this, as well as for improvements to the service. If you have
anything you want to try out, and want to talk to us about it, please email
us at labs at metacarta dot com. If you’d rather just start using the
service, go right ahead: there are no limitations, other than that you
understand the service is currently experimental and may dissappear at
any time.
I look forward to seeing the maps that people upload.
– Christopher Schmidt, Developer, MetaCarta Labs