优化脚本

Question

我正在处理巨大土地覆盖上的高分辨率栅格数据。 我已经实现了我设定的目标（就脚本结果而言）并且它在小型光栅文件上运行良好，但是将其应用于大型光栅文件时需要很长时间。 工作流程是这样的：

1. 从 DEM 栅格文件获取坡向栅格和坡度栅格
2. 将坡向和坡度栅格转换为多边形图层
3. 导出重要的某些坡向和坡度组合
4. 将导出的合并到最后一层，在我的例子中代表“禁区”。

代码：

import pandas as pd
from os.path import join, normpath
import time

start = time.time()

path = 'C:/Users/tlind/Dropbox/Documents/Temp/'

# iface.addRasterLayer(path+'riktigk_raster.tif')

processing.run("native:slope", {'INPUT':path+'riktig.tif','Z_FACTOR':1,'OUTPUT':path+'slope.tif'})
# iface.addRasterLayer(path+'slope.tif')
#
processing.run("native:aspect", {'INPUT':path+'riktig.tif','Z_FACTOR':1,'OUTPUT':path+'aspect.tif'})
# iface.addRasterLayer(path+'aspect.tif')

processing.run("gdal:merge", {'INPUT':['C:/Users/tlind/Dropbox/Documents/Temp/aspect.tif','C:/Users/tlind/Dropbox/Documents/Temp/slope.tif'],'PCT':True,'SEPARATE':True,'NODATA_INPUT':None,'NODATA_OUTPUT':None,'OPTIONS':'','EXTRA':'','DATA_TYPE':5,'OUTPUT':path+'merge.tif'})

processing.run("native:pixelstopolygons", {'INPUT_RASTER':path+'merge.tif','RASTER_BAND':1,'FIELD_NAME':'ASPECT','OUTPUT':path+'aspect.shp'})

processing.run("native:pixelstopolygons", {'INPUT_RASTER':path+'merge.tif','RASTER_BAND':2,'FIELD_NAME':'SLOPE','OUTPUT':path+'slope.shp'})

aspect_layer = iface.addVectorLayer(path+'aspect.shp', "", "ogr")
slope_layer = iface.addVectorLayer(path+'slope.shp', "", "ogr")

pv_apsect = aspect_layer.dataProvider()
pv_apsect.addAttributes([QgsField('ID', QVariant.Double)])

aspect_layer.updateFields()

expression = QgsExpression('$id')

context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(aspect_layer))

with edit(aspect_layer):
    for f in aspect_layer.getFeatures():
        context.setFeature(f)
        f['ID'] = expression.evaluate(context)
        aspect_layer.updateFeature(f)
    
pv_slope = slope_layer.dataProvider()
pv_slope.addAttributes([QgsField('ID', QVariant.Double)])

slope_layer.updateFields()

expression = QgsExpression('$id')

context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(slope_layer))

with edit(slope_layer):
    for f in slope_layer.getFeatures():
        context.setFeature(f)
        f['ID'] = expression.evaluate(context)
        slope_layer.updateFeature(f)
    
processing.run("native:joinattributestable", {'INPUT':path+'aspect.shp','FIELD':'ID','INPUT_2':path+'slope.shp','FIELD_2':'ID','FIELDS_TO_COPY':[],'METHOD':1,'DISCARD_NONMATCHING':False,'PREFIX':'','OUTPUT':path+'aspect_slope.shp'})
aspect_slope_layer = iface.addVectorLayer(path+'aspect_slope.shp', "", "ogr")

slope_list = [4.2, 4.6, 5.1, 5.7, 6.4, 7, 9.5, 12, 15, 18.2, 22.5, 30]
aspect_list_1 = [[0, 15], [15,30], [30, 45], [45, 60], [60, 75], [75, 90], [90, 105], [105, 120], [120, 135], [135, 150], [150, 165], [165, 180]]
aspect_list_2 = [[180, 195], [195, 210], [210, 225], [225, 240], [240, 255], [255, 270], [270, 285], [285, 300], [300, 315], [315, 330], [330, 345], [345, 360]]
# aspect_list_3 = aspect_list_1+aspect_list_2

for i in range(len(slope_list)):
    aspect_list_1[i].append(slope_list[i])

for i in range(len(slope_list)):
    aspect_list_2[i].append(slope_list[::-1][i])

for aspect_interval in aspect_list_1:
        start = aspect_interval[0] 
        end = aspect_interval[1]
        slope_loop = aspect_interval[2]
        aspect_slope_layer.selectByExpression('"ASPECT">'+str(start)+' and "ASPECT"<='+str(end)+' and "SLOPE">='+str(slope_loop))
        QgsVectorFileWriter.writeAsVectorFormat(aspect_slope_layer, str(path)+'aspect_slope_'+str(start)+'-'+str(end)+'.shp', "UTF-8", aspect_slope_layer.crs(), "ESRI Shapefile", onlySelected=True)
        # iface.addVectorLayer(str(path)+'aspect_slope_'+str(start)+'-'+str(end)+'.shp', "", "ogr")

for aspect_interval in aspect_list_2:
        start = aspect_interval[0] 
        end = aspect_interval[1]
        slope_loop = aspect_interval[2]
        aspect_slope_layer.selectByExpression('"ASPECT">'+str(start)+' and "ASPECT"<='+str(end)+' and "SLOPE">='+str(slope_loop))
        QgsVectorFileWriter.writeAsVectorFormat(aspect_slope_layer, str(path)+'aspect_slope_'+str(start)+'-'+str(end)+'.shp', "UTF-8", aspect_slope_layer.crs(), "ESRI Shapefile", onlySelected=True)
        # iface.addVectorLayer(str(path)+'aspect_slope_'+str(start)+'-'+str(end)+'.shp', "", "ogr")
    
processing.run("native:mergevectorlayers", {'LAYERS':['C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_0-15.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_105-120.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_120-135.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_135-150.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_15-30.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_150-165.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_165-180.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_180-195.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_195-210.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_210-225.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_225-240.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_240-255.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_255-270.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_270-285.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_285-300.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_30-45.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_300-315.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_315-330.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_330-345.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_345-360.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_45-60.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_60-75.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_75-90.shp','C:/Users/tlind/Dropbox/Documents/Temp/aspect_slope_90-105.shp'],'CRS':None,'OUTPUT':str(path)+'aspect_slope_final.shp'})
iface.addVectorLayer(str(path)+'aspect_slope_final.shp', "", "ogr")

end = time.time()
print("Elapsed time:", (end-start)/60, "minutes.")

直到从栅格创建多边形层是相当快的。 添加 ID 字段是一件相当耗时的事情。 是否可以通过从一开始就添加多个字段来从光栅生成矢量，而不是之后必须手动进行？ 在这个例子中。 有'FIELD_NAME'：'ASPECT'。

processing.run("native:pixelstopolygons", {'INPUT_RASTER':path+'merge.tif','RASTER_BAND':1,'FIELD_NAME':'ASPECT','OUTPUT':path+'aspect.shp'})

当谈到实际的光栅到像素时，没有太多可做的。 这需要时间。

在结束循环中，导出选定的多边形。 是否可以进行完整的选择并且可能只进行一次导出？ 我想这也会加快速度。

另一个稍微不相关的问题是，是否可以通过让 QGIS 使用更多的 CPU 来加快速度？ 目前我认为它只在一个核心上冲浪。

Answer 1

我设法很好地优化了它！

首先更换所有

pv_apsect = aspect_layer.dataProvider()
pv_apsect.addAttributes([QgsField('ID', QVariant.Double)])

aspect_layer.updateFields()

expression = QgsExpression('$id')

context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(aspect_layer))

with edit(aspect_layer):
    for f in aspect_layer.getFeatures():
        context.setFeature(f)
        f['ID'] = expression.evaluate(context)
        aspect_layer.updateFeature(f)
    
pv_slope = slope_layer.dataProvider()
pv_slope.addAttributes([QgsField('ID', QVariant.Double)])

slope_layer.updateFields()

expression = QgsExpression('$id')

context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(slope_layer))

with edit(slope_layer):
    for f in slope_layer.getFeatures():
        context.setFeature(f)
        f['ID'] = expression.evaluate(context)
        slope_layer.updateFeature(f)
    
processing.run("native:joinattributestable", {'INPUT':path+'aspect.shp','FIELD':'ID','INPUT_2':path+'slope.shp','FIELD_2':'ID','FIELDS_TO_COPY':[],'METHOD':1,'DISCARD_NONMATCHING':False,'PREFIX':'','OUTPUT':path+'aspect_slope.shp'})

简单地

processing.run("native:joinattributesbylocation", {'INPUT':path+'slope.shp','PREDICATE':[5],'JOIN':path+'aspect.shp','JOIN_FIELDS':[],'METHOD':0,'DISCARD_NONMATCHING':False,'PREFIX':'','OUTPUT':path+'aspect_slope.shp'})

另外，我在形状文件中添加了一个空间索引

processing.run("native:createspatialindex", {'INPUT':'LAYER'})

从 28 小时的过程变成了 50 分钟的过程。