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QgsGeometry.length() giving wrong result?

Is it possible to programmatically add calculated fields?Calculate line lengths with Python in QGISWhy can't I multiply values from column named 'length' using field calculator via PyQGIS?Why would having a reference string “boh@10” cause a crash?new vector layer's field based on other layers attributes - “for”cycle errorCalculate Expression with Python?how to get first and last point of MultiLineString-objects in qgis3Adding new field with expression in pyqgis

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty{ margin-bottom:0;
}

3

I have a vector layer with Multilinestrings and used the field calculator to add a field ("length") with the calculated $length of each feature. I know the field values are correct. Now, if I calculate the length of the features with PyQGIS, I get different, wrong results. Does anybody know, what could be the reason? I don't know where to start looking for the error.

Code example:

layer = iface.activeLayer()

feature = next(layer.getFeatures())

print(feature.geometry().length() / feature['length'])

Result: 1.67

pyqgis qgis-3

share|improve this question

edited 2 days ago

PolyGeo♦

54.9k1717 gold badges8686 silver badges258258 bronze badges

asked Aug 16 at 10:53

lueholueho

11899 bronze badges

add a comment | 

3

I have a vector layer with Multilinestrings and used the field calculator to add a field ("length") with the calculated $length of each feature. I know the field values are correct. Now, if I calculate the length of the features with PyQGIS, I get different, wrong results. Does anybody know, what could be the reason? I don't know where to start looking for the error.

Code example:

layer = iface.activeLayer()

feature = next(layer.getFeatures())

print(feature.geometry().length() / feature['length'])

Result: 1.67

pyqgis qgis-3

share|improve this question

edited 2 days ago

PolyGeo♦

54.9k1717 gold badges8686 silver badges258258 bronze badges

asked Aug 16 at 10:53

lueholueho

11899 bronze badges

add a comment | 

I have a vector layer with Multilinestrings and used the field calculator to add a field ("length") with the calculated $length of each feature. I know the field values are correct. Now, if I calculate the length of the features with PyQGIS, I get different, wrong results. Does anybody know, what could be the reason? I don't know where to start looking for the error.

Code example:

layer = iface.activeLayer()

feature = next(layer.getFeatures())

print(feature.geometry().length() / feature['length'])

Result: 1.67

pyqgis qgis-3

share|improve this question

edited 2 days ago

PolyGeo♦

54.9k1717 gold badges8686 silver badges258258 bronze badges

asked Aug 16 at 10:53

lueholueho

11899 bronze badges

layer = iface.activeLayer()

feature = next(layer.getFeatures())

print(feature.geometry().length() / feature['length'])

share|improve this question

edited 2 days ago

PolyGeo♦

54.9k1717 gold badges8686 silver badges258258 bronze badges

asked Aug 16 at 10:53

lueholueho

11899 bronze badges

share|improve this question

edited 2 days ago

PolyGeo♦

54.9k1717 gold badges8686 silver badges258258 bronze badges

asked Aug 16 at 10:53

lueholueho

11899 bronze badges

edited 2 days ago

PolyGeo♦

54.9k1717 gold badges8686 silver badges258258 bronze badges

edited 2 days ago

PolyGeo♦

54.9k1717 gold badges8686 silver badges258258 bronze badges

asked Aug 16 at 10:53

lueholueho

11899 bronze badges

asked Aug 16 at 10:53

lueholueho

11899 bronze badges

2 Answers
2

active

oldest

votes

3

The $length expression function states that

The length calculated by this function respects both the current
project's ellipsoid setting and distance unit settings. For example,
if an ellipsoid has been set for the project then the calculated
length will be ellipsoidal, and if no ellipsoid is set then the
calculated length will be planimetric.

QgsGeometry.length on the other hand says

Returns the length of geometry using GEOS

Sadly this is not very helpful if you don't know what GEOS means. GEOS is the underlying geometry engine and it has no idea about coordinate systems, ellipsoids and all that spatial stuff. It is purely about geometry. This means QgsGeometry.length will consider all coordinate values of the geometry as flat cartesian coordinates. You might know they are geographic/ellipsoidal, GEOS does not.

If you are in a length-preserving projection with meaningful units (meters), the result will (almost) be the measurement as if done in the real world. But in all other cases, the results from this function are rather useless.

share|improve this answer

answered Aug 16 at 11:05

bugmenot123bugmenot123

4,4521414 silver badges4343 bronze badges

add a comment | 

0

Your layer's coordinate system is probably a Geographic one (meaning Ellipsoidal here), therefore the distance calculated in degree dimension. To solve this issue, first you have to a pick a proper Projected coordinate system, and save the layer in that coordinate system (Save as, change CRS).

You can find a proper Projected coordinate system at https://epsg.io, e.g. search for your country.

After the projection, the lenght() method's result will be fine.

share|improve this answer

answered Aug 16 at 11:17

pnzpnz

90644 silver badges1717 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
  
  – bugmenot123
  Aug 16 at 13:19
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Maybe try it...
  
  – pnz
  23 hours ago
  

  
  
  
  

add a comment | 

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3

The $length expression function states that

The length calculated by this function respects both the current
project's ellipsoid setting and distance unit settings. For example,
if an ellipsoid has been set for the project then the calculated
length will be ellipsoidal, and if no ellipsoid is set then the
calculated length will be planimetric.

QgsGeometry.length on the other hand says

Returns the length of geometry using GEOS

Sadly this is not very helpful if you don't know what GEOS means. GEOS is the underlying geometry engine and it has no idea about coordinate systems, ellipsoids and all that spatial stuff. It is purely about geometry. This means QgsGeometry.length will consider all coordinate values of the geometry as flat cartesian coordinates. You might know they are geographic/ellipsoidal, GEOS does not.

If you are in a length-preserving projection with meaningful units (meters), the result will (almost) be the measurement as if done in the real world. But in all other cases, the results from this function are rather useless.

share|improve this answer

answered Aug 16 at 11:05

bugmenot123bugmenot123

4,4521414 silver badges4343 bronze badges

add a comment | 

3

The $length expression function states that

The length calculated by this function respects both the current
project's ellipsoid setting and distance unit settings. For example,
if an ellipsoid has been set for the project then the calculated
length will be ellipsoidal, and if no ellipsoid is set then the
calculated length will be planimetric.

QgsGeometry.length on the other hand says

Returns the length of geometry using GEOS

Sadly this is not very helpful if you don't know what GEOS means. GEOS is the underlying geometry engine and it has no idea about coordinate systems, ellipsoids and all that spatial stuff. It is purely about geometry. This means QgsGeometry.length will consider all coordinate values of the geometry as flat cartesian coordinates. You might know they are geographic/ellipsoidal, GEOS does not.

If you are in a length-preserving projection with meaningful units (meters), the result will (almost) be the measurement as if done in the real world. But in all other cases, the results from this function are rather useless.

share|improve this answer

answered Aug 16 at 11:05

bugmenot123bugmenot123

4,4521414 silver badges4343 bronze badges

add a comment | 

The $length expression function states that

The length calculated by this function respects both the current
project's ellipsoid setting and distance unit settings. For example,
if an ellipsoid has been set for the project then the calculated
length will be ellipsoidal, and if no ellipsoid is set then the
calculated length will be planimetric.

QgsGeometry.length on the other hand says

Returns the length of geometry using GEOS

Sadly this is not very helpful if you don't know what GEOS means. GEOS is the underlying geometry engine and it has no idea about coordinate systems, ellipsoids and all that spatial stuff. It is purely about geometry. This means QgsGeometry.length will consider all coordinate values of the geometry as flat cartesian coordinates. You might know they are geographic/ellipsoidal, GEOS does not.

If you are in a length-preserving projection with meaningful units (meters), the result will (almost) be the measurement as if done in the real world. But in all other cases, the results from this function are rather useless.

share|improve this answer

answered Aug 16 at 11:05

bugmenot123bugmenot123

4,4521414 silver badges4343 bronze badges

share|improve this answer

answered Aug 16 at 11:05

bugmenot123bugmenot123

4,4521414 silver badges4343 bronze badges

answered Aug 16 at 11:05

bugmenot123bugmenot123

4,4521414 silver badges4343 bronze badges

answered Aug 16 at 11:05

bugmenot123bugmenot123

4,4521414 silver badges4343 bronze badges

0

Your layer's coordinate system is probably a Geographic one (meaning Ellipsoidal here), therefore the distance calculated in degree dimension. To solve this issue, first you have to a pick a proper Projected coordinate system, and save the layer in that coordinate system (Save as, change CRS).

You can find a proper Projected coordinate system at https://epsg.io, e.g. search for your country.

After the projection, the lenght() method's result will be fine.

share|improve this answer

answered Aug 16 at 11:17

pnzpnz

90644 silver badges1717 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
  
  – bugmenot123
  Aug 16 at 13:19
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Maybe try it...
  
  – pnz
  23 hours ago
  

  
  
  
  

add a comment | 

0

Your layer's coordinate system is probably a Geographic one (meaning Ellipsoidal here), therefore the distance calculated in degree dimension. To solve this issue, first you have to a pick a proper Projected coordinate system, and save the layer in that coordinate system (Save as, change CRS).

You can find a proper Projected coordinate system at https://epsg.io, e.g. search for your country.

After the projection, the lenght() method's result will be fine.

share|improve this answer

answered Aug 16 at 11:17

pnzpnz

90644 silver badges1717 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I sure hope that QGIS provides functions for calculating "true" lengths without having to reproject manually.
  
  – bugmenot123
  Aug 16 at 13:19
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Maybe try it...
  
  – pnz
  23 hours ago
  

  
  
  
  

add a comment | 

Your layer's coordinate system is probably a Geographic one (meaning Ellipsoidal here), therefore the distance calculated in degree dimension. To solve this issue, first you have to a pick a proper Projected coordinate system, and save the layer in that coordinate system (Save as, change CRS).

You can find a proper Projected coordinate system at https://epsg.io, e.g. search for your country.

After the projection, the lenght() method's result will be fine.

share|improve this answer

answered Aug 16 at 11:17

pnzpnz

90644 silver badges1717 bronze badges

share|improve this answer

answered Aug 16 at 11:17

pnzpnz

90644 silver badges1717 bronze badges

answered Aug 16 at 11:17

pnzpnz

90644 silver badges1717 bronze badges

answered Aug 16 at 11:17

pnzpnz

90644 silver badges1717 bronze badges