layer对象有一个方法叫setattributefilter()可以将layer中符合某一条件的feature过滤出来。设定了filter之后就可以用getnextfeature()方法依次取出符合条件的feature了。setattributefilter(none)可以清楚一个filter。例如
>>> layer.getfeaturecount()
42
>>> layer.setattributefilter(“cover = ‘shrubs'”)
>>> layer.getfeaturecount()
6
>>> layer.setattributefilter(none)
>>> layer.getfeaturecount()
42
空间过滤器spatial filters
有两种。一种是setspatialfilter(),过滤某一类型的feature,例如参数中填polygon,就是选出layer中的所有polygon
另外还有setspatialfilterrect(, , , ),参数输入四个坐标,可以选中方框内的feature
setspatialfilter(none)一样是清空空间属性过滤器。
例如下面这段代码,layerareas 是polygon,layersites是point
>>> featareas = layerareas.getnextfeature()
>>> poly = featareas.getgeometryref()
>>> layersites.getfeaturecount()
42
>>> layersites.setspatialfilter(poly)
>>> layersites.getfeaturecount()>>> layersites.getfeaturecount()
33
>>> layersites.setspatialfilterrect(460000, 4590000, 490000, 4600000)
>>> layersites.getfeaturecount()
4
>>> layersites.setspatialfilter(none)
>>> layersites.getfeaturecount()
42
还有更复杂的filter,例如执行sql查询语句executesql(),凭借sql的强大功能,可以执行更复杂的任务,例如下面这段代码,就是选择cover类型为grass的feature,并且按id号降序排列。
result = dssites.executesql(“select * from sites where cover = ‘grass’ order by id desc”)
resultfeat = result.getnextfeature()
while resultfeat :
print resultfeat.getfield(‘id’)print resultfeat.getfield(‘id’)
resultfeat = result.getnextfeature()
dssites.releaseresultset(result)
42
40
:
4
最后一句releaseresultset()是将查询结果释放,在执行下一条sql语句之前一定要先释放。
下面的例子,统计了cover为grass的所有feature的数目
>>> result = dssites.executesql(“select count(*) from sites where cover = ‘grass'”)
>>> result.getfeaturecount()
11
>>> result.getfeature(0).getfield(0)
11
>>> dssites.releaseresultset(result)
列出所有不同的cover类型
result = ds.executesql(“select distinct cover from sites”)
resultfeat = result.getnextfeature()
while resultfeat:
print resultfeat.getfield(0)
resultfeat = result.getnextfeature()
ds.releaseresultset(result)
shrubs
trees
rocks
grass
bare
water
统计每种cover类型各有多少个feature
coverlayer = ds.executesql(‘select distinct cover from sites’)
coverfeat = coverlayer.getnextfeature()
while coverfeat:
cntlayer = ds.executesql(“select count(*) from sites where cover = ‘ “ + coverfeat.getfield(0) + “ ‘ “)
print coverfeat.getfield(0) + ‘ ‘ +print coverfeat.getfield(0) + ‘ ‘ + cntlayer.getfeature(0).getfieldasstring(0)
ds.releaseresultset(cntlayer)
coverfeat = coverlayer.getnextfeature()
ds.releaseresultset(coverlayer)
shrubs 6
trees 11
rocks 6
grass 11
bare 6
water 2
intersect判断两个要素是否相交
poly2.intersect(poly1)
返回0表示不相交,返回1表示相交
disjoint判断两个要素是否不相交poly2.disjoint(poly1)
返回1表示不相交,返回0表示相交,跟intersect正好相反
touch表示相邻(擦边)
poly2.touches(poly1)
返回0表示不擦边,返回1表示擦边
crosses穿越,一般是一条线穿过一个多边形
poly2.crosses(line)
返回0表示不穿过,返回1表示穿过
within包含,一个要素完全被另一个要素圈起来了
ptb.within(poly1)
返回0表示点在多边形外,返回1表示点在多边形内
contains包含,跟within正好相反
poly1.contains(ptb)
就是把主调对象和参数换一下啦
overlaps重叠,好像只有两个多边形之间才能overlap
poly2.overlaps(poly3)
返回0表示不重叠,返回1表示重叠
下面看看简单的地理数据处理geoprocessing
多边形的:
交:poly3.intersection(poly2)
并:poly3.union(poly2)
差:poly3.difference(poly2)
补:poly3.symmetricdifference(poly2)
geometry的:
.buffer() 给geometry加buffer,就是把点线变成多边形,变粗了
.equal() 两个geometry相等吗?
.distance() 返回两个geometry之间的最短距离
.getenvelope() 信封,有意思,其实就是用一个方框框住这个几何形状,返回四个角的坐标(minx, maxx, miny, maxy)
python的函数function,异常exception和模块module可以从任何一本python教材上找到,在此不多赘述
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