優化腳本

Question

我正在處理巨大土地覆蓋上的高分辨率柵格數據。 我已經實現了我設定的目標（就腳本結果而言）並且它在小型光柵文件上運行良好，但是將其應用於大型光柵文件時需要很長時間。 工作流程是這樣的：

1. 從 DEM 柵格文件獲取坡向柵格和坡度柵格
2. 將坡向和坡度柵格轉換為多邊形圖層
3. 導出重要的某些坡向和坡度組合
4. 將導出的合並到最后一層，在我的例子中代表“禁區”。

代碼：

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.")

直到從柵格創建多邊形層是相當快的。 添加 ID 字段是一件相當耗時的事情。 是否可以通過從一開始就添加多個字段來從光柵生成矢量，而不是之后必須手動進行？ 在這個例子中。 有'FIELD_NAME'：'ASPECT'。

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

當談到實際的光柵到像素時，沒有太多可做的。 這需要時間。

在結束循環中，導出選定的多邊形。 是否可以進行完整的選擇並且可能只進行一次導出？ 我想這也會加快速度。

另一個稍微不相關的問題是，是否可以通過讓 QGIS 使用更多的 CPU 來加快速度？ 目前我認為它只在一個核心上沖浪。

Answer 1

我設法很好地優化了它！

首先更換所有

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'})

簡單地

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，我在形狀文件中添加了一個空間索引

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

從 28 小時的過程變成了 50 分鍾的過程。