Chris hill rps_postgis_threeoutoffouraintbad_20150505_1
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- PostGIS allows spatial and attribute querying together, advanced joins, aggregation functions, scripting in SQL for repeatability, good raster/vector integration, and linking to other languages like Python and R. - It may take longer than desktop GIS for single tasks but enables automation through SQL scripts. - The document discusses fuzzy polygon matching, measuring closest distances, creating simple transects with translation, rotation, and directionality, and provides examples of PostGIS functions like ST_Intersects and ST_Distance.
Chris hill rps_postgis_threeoutoffouraintbad_20150505_1
- 1. rpsgroup.com/uk PostGIS: three out of four ain’t bad Chris Hill 05 May 2015
- 2. A slide from 11 years ago… PostgreSQL/PostGIS ✓ It’s a relational database ✓ We’ll look at this… ✘ Use QGIS or st_astext() ✓ We’ll look at this too…
- 3. Why you should use PostGIS over a desktop GIS • Spatial and attribute querying together • Permits advanced joins • Aggregate (group by) functions • Transcript of all work done • Scripting in SQL – Repeatability – Enforced documenting of processes • Good raster/vector integration • Can link to R, Python, QGIS… Will it take longer than a desktop GIS? – Possibly, if you only do something once
- 5. st_intersects 703 st_transform 461 st_union 454 st_length 435 st_buffer 386 st_area 362 st_centroid 307 st_setsrid 280 st_y 257 st_startpoint 255 st_x 244 st_endpoint 232 st_makevalid 225 st_buildarea 212 st_dump 198 st_intersection 180 st_collect 176 st_azimuth 168 st_makeline 143 st_makepoint 121 Analysis functions Management functions My top 20 PostGIS functions
- 7. Fuzzy polygon matching 2 --This works nicely (in this case) st_intersects(st_centroid(a.geom),b.geom); --But for holes or very irregular polygons try st_intersects(st_pointonsurface(a.geom),b.geom); --first attempt (will be greedy) st_intersects(a.geom,b.geom); Perhaps use the logic of all three options? select a.poly_id, b.poly_id from dodgypolys1 a, dodgypolys2 b where
- 8. Measuring closest distances 1 Copyright Scottish Natural Heritage Contains Ordnance Survey data © Crown copyright and database right 2015 Contains Ordnance Survey data © Crown copyright and database right 2015
- 9. Measuring closest distances 2 Optionally create a view UNIONing all target polygons together with turbines1 as (select st_union(geom) as geom from turbines) copy ( select b.designation, b.name, st_distance(a.geom, b.geom) from turbines1 a, designations b where st_dwithin(a.geom,b.geom,20000) order by 3 ) to E'C:designationsWithin20km.csv' with csv header;
- 10. Create simple transects 1
- 11. Create simple transects 2 with linetemplate as (select st_geomFromText('LINESTRING(0 0,0 10)') as geom), x_intervals as (select generate_series(0,10,2) as deltax) create table transects1 as select deltax, st_translate(geom,deltax,0) as geom from linetemplate cross join x_intervals;
- 12. Create simple transects 3: add rotation
- 13. Create simple transects 4: add rotation create table transects2 as with lines1 as (select st_union(geom)as geom from transects1) select (st_dump( st_rotate(geom,radians(45), st_centroid(geom)) )).geom as geom from lines1; alter table transects2 add column line_id serial;
- 14. Create simple transects 5: add directionality
- 15. Create simple transects 6: add directionality update transects2 set geom = st_reverse(geom) where line_id % 2 = 0;
- 17. Homework: to make the rays you might use… st_makeline() st_length() generate_series() ST_Line_Interpolate_Point() st_exteriorring() st_buffer() CROSS JOIN st_makepoint()