On Sun, Aug 23, 2009 at 12:08 AM, Itzchak Raiskin<itzchak.rais...@gmail.com> wrote: > Hi > I want to use SQLite in a GIS application where I create a database > containing terrain data (coordinates, height). > I would like to query this database with start and end points of a line and > get a vector with all heights point along this line. > I can, of course create a query for each point along the line, but this will > be very time consuming as I have hundreds of lines with hundreds of points. > Any suggestions? >
The problem to solve is that you have a discontinuous surface (discrete points) with height values. You want to do a line-on-poly overlay and find the surface values that intersect with the line. Of course, the line may or may not intersect with any of your stored points, so you have to do some interpolation. If you can "fill" all the coords on a surface, typically done by an image where every pixel holds the height value, you can find all the points that coincide with a line. If a line is expressed by (y = ax + c), you need to find all x,y that will satisfy that equation. So, SELECT x, y, z FROM terrain WHERE a*x - y + c = 0 But you can make the search more efficient by only searching within the bounding box of the line, so, assuming xmin, ymin, xmax, ymax are the end points of the line SELECT x, y, z FROM terrain WHERE a*x - y + c = 0 AND x BETWEEN xmin AND xmax AND y BETWEEN ymin AND ymax In other words, Kit gave you the answer already. I hope my explanation helped flesh it out more. You can use your own line equation in the SELECT statement above. -- Puneet Kishor http://www.punkish.org Carbon Model http://carbonmodel.org Charter Member, Open Source Geospatial Foundation http://www.osgeo.org Science Commons Fellow, http://sciencecommons.org/about/whoweare/kishor Nelson Institute, UW-Madison http://www.nelson.wisc.edu ----------------------------------------------------------------------- Assertions are politics; backing up assertions with evidence is science ======================================================================= Sent from Madison, WI, United States _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
