官术网_书友最值得收藏!

Calculating the area of a polygon

This recipe simply measures the area of a polygon.

Getting ready

For this recipe, we'll use a single-feature polygon shapefile, which you can download from https://geospatialpython.googlecode.com/files/Mississippi.zip

Unzip the shapefile and put it in a directory named qgis_data/ms within your root or home directory.

How to do it...

Perform the following steps to measure the area of a large polygon:

  1. First, import the QGIS constants library, as follows:
    from qgis.core import QGis
    
  2. Load the layer:
    lyr = QgsVectorLayer("/qgis_data/ms/mississippi.shp", "Mississippi", "ogr")
    
  3. Access the layer's features:
    fts = lyr.getFeatures()
    
  4. Get the boundary feature:
    boundary = fts.next()
    
  5. Create the measurement object instance:
    d = QgsDistanceArea()
    
  6. Pass the polygon list to the measureArea() method:
    m = d.measurePolygon(boundary.geometry().asPolygon()[0])
    
  7. Convert the measurement from decimal degrees to miles:
    d.convertMeasurement(m, QGis.Degrees, QGis.NauticalMiles, True) 
    
  8. Verify that your output looks similar to the following:
    (42955.47889640281, 7)
    

How it works...

PyQIS has no measureArea() method, but it has a measurePolygon() method in the QgsDistanceArea object. The method accepts a list of points. In this case, when we convert the measurement output from decimal degrees to miles, we also specify True in the convertMeasurement() method so that QGIS knows that it is an area calculation. Note that when we get the boundary geometry as a polygon, we use an index of 0, suggesting that there is more than one polygon. A polygon geometry can have inner rings, which are specified as additional polygons. The outermost ring, in this case the only ring, is the first polygon.

主站蜘蛛池模板: 和林格尔县| 吉安市| 攀枝花市| 马尔康县| 日照市| 图木舒克市| 聊城市| 湛江市| 宁都县| 靖宇县| 册亨县| 遵义县| 永修县| 中西区| 福鼎市| 甘德县| 定州市| 濮阳县| 博白县| 靖安县| 磐安县| 剑阁县| 台湾省| 额济纳旗| 阳春市| 永康市| 天镇县| 白河县| 封丘县| 芮城县| 焉耆| 大洼县| 新乡市| 杨浦区| 威远县| 浪卡子县| 体育| 新民市| 南京市| 逊克县| 海林市|