I figured it was at least as relevant here, so here's a copy. Brent Wood
--- Kurt Schwehr <[EMAIL PROTECTED]> wrote: > > Hi Brent, > > That sounds much like what I am trying to do. I have the data in GMT > multisegment lines, PostgreSQL/PostGIS, and SQLite for various tasks. > Do you have a paper or tech report on what you did? Being written at present. A client report for a government dept, whether it makes it out of the grey literature is yet to be decided. The summary below should cover it for you though. I used a script to generate 5000m x 5000m cells in a custom local equal area projection created in the spatial_ref_sys table, then added anothe EPSG:4326 polygon column to this table & updated this with the lat/long transform() of the equal area cells, so I had equal area cells in a lat long projection to work with my lat long trackline data. Tracks were then buffered to polygons (to give swept areas) using a transform() to equal area to buffer(), then transform() back to lat/long, ie: transform(buffer(transform(trackline,27201),doorspread/2, 8)), 4326) & these areas were joined (cropped) at cell boundaries so each track was divided into cell based sections. This allows easy access to no of tracks/cell & aggregate swept areas/cell (which can exceed the cell area where a cell is encountered often). The merged area/cell to give the % of each cell that has been fished is done with geomunion() to dissolve cell-trackline boundaries to give a single multipolygon per cell of the fished region in each cell. This gave a few problems, which an update to geos v3 (latest RC version) pretty much remedied. With up to thousands of polygons in a cell to merge, it wasn't pretty topologically speaking. Once this geoprocessing was complete, it was just basic queries to sumarise/analyse the data, & a script to loop through species, years, methods etc to use GMT to iteratively & semi automagically generate some 2000 maps of appropriate subsets of the data extracted from the database on the fly as it went. All done on a dual core Athlon64 box with the data on striped WD Raptor SATA drives & 4Gb memory with 64bit OpenSuse 10.2 & everything copiled from scratch. The Raptors gave about 150Mb/sec reads, which helps a lot :-) My problem now is how to best manage this number of maps in the report :-) Hope this helps, if you need more info, let me know. Cheers, Brent > > Thanks, > -kurt > > > > > > Hi Kurt, > > > > Another option for you. > > > > I have done this exercise with 1,500,000 ship tracks to model fishing > effort. > > I've used GMT as the cartographic tool, but the data was stored & the cell > > counts done in PostGIS. Being able to group data by vessel category, target > > species, etc, was also necessary, and SQL with PostGIS extensions provides > lots > > of functionality and flexibility to manage/query/overlay the data. > > > > I also turned tracklines into polygons based on wingspread/doorspread for > > trawlers. so swept area polygons could be analysed by cell, not just the > > tracklines. > > > > > > Cheers, > > > > Brent Wood > > > > > > > > _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
