Often when processing satellite imagery clipping the data to an AOI makes sense. If you just want to build up a map of one area or use it for statistics then clipping is a standard part of a GIS process. Finding reliable data to clip to, or search with, is harder than it should be. Let’s say for instance you wanted a boundary of Beverly Hills in California.
The first result on Google is this http://www.zipmap.net/California/Los_Angeles_County/Beverly_Hills.htm which does give me the answer but I cannot seem to be able to do anything with the data. Next result wants me to create an account to login, the next Wikipedia and so on.
I change my search to “beverly hills california shapefile”
Getting closer… Firstly I get an interactive map that does seem to have a download, secondly a link to arcgis.com – sadly this is just a shapefile of LA, with Beverly Hills cut out. At this point I could create a polygon from this donut polygon, but really I just want Beverly Hills area.
Next up the TIGER/Line dataset; this one contains 490,385 rows of data.
Beverly Hills must be in there somewhere, right? This shapefile is also a line shapefile and the boundaries seem segmented when I look at the data.
Finally, the planning website planning.lacounty.gov/gis gets me (via the online portal) to the boundary.
The fourth result on my second Google search got me to the boundary. In 2017 my expectations are that this should be much simpler and while local/county/state portals can provide excellent information I often want to be able to search on a worldwide basis.
Why is it that when I search on Google Maps I get a boundary first time! If only I could use this data!
Well, with OpenStreetMap I can!
search for a name or address(forward search) or look up data by its geographic coordinate(reverse search). Each result comes with a link to a details page where you can inspect what data about the object is saved in the database and investigate how the address of the object has been computed.
Nice one. So how to get this boundary out? Click on details
You need the ID I have highlighted in the red box. Navigate to this page http://polygons.openstreetmap.fr/index.py and paste the ID into the box and submit query.
and choose which polygon you want: it can also be simplified (which is perfect for clipping satellite data!). GeoJSON works well for me. Here is my clipped satellite image.
So next time you are looking for a generalised polygon/boundary try OpenStreetMap!
I have grouped all my previous blogs (technical stuff / tutorials / opinions / ideas) are here http://gis.acgeospatial.co.uk
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