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書名： QGIS Python Programming Cookbook
作者名： Joel Lawhead
本章字數： 313字
更新時間： 2021-07-23 19:48:54

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://geospatialpython.googlecode.com/svn/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 their distance:

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:
```
d.convertMeasurement(m, 2, 0, False)
```
Ensure that your Python console output looks similar to this 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 verses a linear measurement.

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

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QGIS Python Programming Cookbook

Measuring the distance between two points

Getting ready

How to do it...

How it works...