Measuring the distance between two points_QGIS Python Programming Cookbook（Second Edition）-QQ阅读男生都市网

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Measuring the distance between two points

In the QgsDistanceArea object, PyQGIS has excellent capabilities for measuring the distance. We'll use this object for several recipes, starting with measuring the distance between two points.

Getting ready

If you don't already have the New York City Museums layer used in the previous recipes in this chapter, download the layer from https://github.com/GeospatialPython/Learn/raw/master/NYC_MUSEUMS_GEO.zip.

Unzip that file and place the shapefile's contents in a directory named nyc within your qgis_data directory, within your root or home directory.

How to do it...

In the following steps, we'll extract the first and last points in the layer's point order and measure the distance between them:

First, import the library that contains the QGIS contents:
```
        from qgis.core import QGis 
```

Then, load the layer:

        lyr = QgsVectorLayer("/qgis_data/nyc/NYC_MUSEUMS_GEO.shp",
                             "Museums", "ogr")

Access the features:
```
        fts = lyr.getFeatures() 
```
Get the first feature:
```
        first = fts.next() 
```
Set a placeholder for the last feature:
```
        last = fts.next() 
```
Iterate through the features until you get the last one:
```
        for f in fts: 
            last = f 
```
Create a measurement object:
```
        d = QgsDistanceArea() 
```

Measure the distance:

        m = d.measureLine(first.geometry().asPoint(),
                          last.geometry().asPoint())

Convert the measurement value from decimal degrees to meters:

        deg = QGis.DecimalDegrees 
        met = QGis.Meters 
        d.convertMeasurement(m, deg, met, False)

Ensure that your Python console output looks similar to the following tuple:
```
        (4401.1622240174165, 0)
```

How it works...

The QgsDistanceArea object accepts different types of geometry as input. In this case, we use two points. The map units for this layer are in decimal degrees, which isn't meaningful for a distance measurement. So, we use the QgsDistanceArea.convertMeasurement() method to covert the output to meters. The parameters for the method are the measurement output, the input units (in decimal degrees), the output units (meters), and a boolean to denote whether this conversion is an area calculation or a linear measurement.

The returned tuple is the measurement value and the units. The value 0 tells us that the output is in meters.