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Advanced GIS Vector Data Analysis: Filters, Functions, and OGR Driver Usage

This week's lesson focuses on using GIS for environmental analysis, specifically filtering and analyzing vector data. Key concepts include defining data structures, working with OGR drivers, and utilizing layers and fields to filter features based on attributes. We explore methods like SetAttributeFilter for querying specific data, implementing spatial filters, and executing SQL queries to manipulate vector layers effectively. This hands-on analysis is crucial for deriving insights from spatial data and making informed environmental decisions.

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Advanced GIS Vector Data Analysis: Filters, Functions, and OGR Driver Usage

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  1. ABT 182 / HYD 182 Environmental Analysis using GIS Week 5-2 Filters & Analysis of vector data Functions & Modules

  2. OGR Driver( definition of data structure ) | ------ Datasource(file / database) | ------ Layer(1 or more) | ------ Field(several) Feature (several) | ------ Geometry Field-values

  3. Attribute filters The Layer object has a method SetAttributeFilter(<where_clause>) >>> layer.GetFeatureCount() 42 >>> layer.SetAttributeFilter("cover = 'shrubs'") >>> layer.GetFeatureCount() 6 >>> layer.SetAttributeFilter(None) >>> layer.GetFeatureCount() 42

  4. Spatial filters • SetSpatialFilter(<geom>) • SetSpatialFilterRect(<minx>, <miny>, <maxx>, <maxy>) featAreas = layerAreas.GetNextFeature() poly = featAreas.GetGeometryRef() layerSites.SetSpatialFilter(poly) layerSites.SetSpatialFilterRect(460000, 4590000, 490000, 4600000) layerSites.SetSpatialFilter(None) See: http://www.gdal.org/ogr/classOGRLayer.html

  5. SQL filters q=("select * from sites where cover = 'grass' order by id desc") layer = datasource.ExecuteSQL(q) layer.GetFeatureCount() layer.GetFeature(0).GetField(0) feat = layer.GetNextFeature() while feat : print feat.GetField('id') feat = layer.GetNextFeature() datasource.ReleaseResultSet(layer) http://www.gdal.org/ogr/ogr_sql.html

  6. Count of each unique value in a variable layer = ds.ExecuteSQL('select distinct cover from sites') feat = layer.GetNextFeature() while feat: q = "select count(*) from sites where cover = '" + coverFeat.GetField(0) + "'" count = ds.ExecuteSQL(q) print feat.GetField(0) + ' ' + cnt.GetFeature(0).GetFieldAsString(0) ds.ReleaseResultSet(count) feat = layer.GetNextFeature() ds.ReleaseResultSet(coverLayer) shrubs 6 trees 11 rocks 6 grass 11 bare 6 water 2

  7. Spatial queries (True / False) point1.Within(poly1) line1.Intersect(line2) line1.Touches(poly2)

  8. poly1 poly2 Spatial queries poly1.Intersection(poly2) poly1.Union(poly2) poly1.Difference(poly2) poly1.SymmetricDifference(poly2)

  9. Spatial queries Buffer a geometry <geom>.Buffer(<distance>) Distance between two geometries <geom1>.Distance(<geom2>) A geometry's extent (xminx, xmax, yminy, ymax) <geom>.GetEnvelope()

  10. Try … finally a = 10 b = 0 try: ds = driver.CreateDataSource(fn) finally: ds.Destroy

  11. Try … except … except … finally import sys, traceback a = 10 b = 0 try: print a / b except 'e1': print a / (b * 2) except: print 'Exception type:', sys.exc_info()[0] print 'Exception value:', sys.exc_info()[1] traceback.print_exc() finally: print 'done'

  12. defadd(a, b): c = a + b return c print add(5, 1) defadd2(a, b=10): c = a + b return c print add2(5) defdistance(x1, y1, x2, y2): y = (y2 - y1)**2 x = (x2 - x1)**2 d = math.sqrt(x + y) return d print distance(0,0,1,1) 1.4142135623730951 Functions

  13. Putting function in a module Put the function in a file e.g. mymod.py import mymod Python will look in: - the directory that the running script is in - then the PYTHONPATH environment variable - then possibly the current working directory - then in the standard library directories (site-packages) import sys sys.path.append('d:/temp') If there is a function myFun inside: mymod.myFun() Import other modules at the top of your module

  14. Glob: Lists files that match a pattern * wildcard for multiple characters ? wildcard for 1 character [] matches character ranges, like [0-9], [a-z], or [a,e,i,o,u] import glob files = glob.glob('*.shp') fn = files[0] newfn = fn.replace('.shp', '_new.shp') prjfn = newfn.replace('.shp', '.prj') newfn = fn[:-4] + '_new.shp' prjfn = newfn[:-4] + '.prj'

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