Measure the distance between two selected locations (in meters) and the direction or angle (in degrees) with respect to the North.
The Measure Distance function uses the coordinate system that is used by a map window:
- when one or more raster maps are displayed in a map window, the map window uses the coordinate system of the georeference of the raster map(s);
- when only vector maps are displayed in a map window, in principle the map window uses the coordinate system of the first vector map;
- however, for vector maps, if you added another coordinate system or another georeference to the map window, by dragging or by choosing Coordinate System from the Options menu,
the newly added coordinate system will be used.
To start the Measure Distance function:
- Choose Measure Distance from the Options menu in a map window, or
- click the Measure Distance
button in the toolbar of the map window.
When the mouse pointer appears as a pair of compasses:
- locate the mouse pointer at the start position from which you wish to measure;
- press and hold down the left mouse button;
- move the mouse pointer to the end position; while moving the mouse pointer, the distance is continuously displayed;
- then, release the left mouse button.
Results in the Measure Distance message box:
From:
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Shows the XY-coordinate or the LatLon-coordinate of the start position.
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To:
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Shows the XY-coordinate or the LatLon-coordinate of the end position.
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Distance on map:
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Shortest distance in meters between start position and end position, calculated in a plane (Euclidean distance).
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Azimuth on map:
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Angle between the line from start position to end position and the XY North of the map (in degrees, clockwise). Tip: You can show the XY-North of a map by adding grid lines to the map window.
- For North oriented maps, e.g. raster maps with a georef corners, and by default for all vector maps:
Start
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End
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Azimuth
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South
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North
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0
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North
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South
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180
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West
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East
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90
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East
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West
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270
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- For satellite images with a georef tiepoints, etc.: the map North can be located anywhere.
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Ellipsoidal Distance:
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Shortest distance in meters (or kilometers) between start position and end position, calculated over the ellipsoid.
- In case the distance is larger than 800 km, the distance will be calculated over the sphere, using the current equatorial radius.
- Also, when system coordinate system LatLon is used, which has no ellipsoid, the distance will be calculated over the sphere (spherical distance).
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Ellipsoidal Azimuth:
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Angle between the line from start position to end position and the geographic North of the map (in degrees, clockwise). Tip: You can show the geographic North of a map by adding a graticule to the map window.
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Scale Factor:
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Indicator of scale distortion, calculated as Distance on map / Ellipsoidal Distance.
For example, when the Scale Factor is 1.001500, it means that the measured Distance on map should be divided by 1.001500 to obtain the true distance (Ellipsoidal Distance).
The Scale Factor depends on the position in the map and on the parameters of the map's projection.
- As the Scale Factor is calculated over the total measured distance, the Scale Factor figure should be regarded as an average.
- The Scale Factor is therefore most reliable when measuring very short distances (in theory, an infinitely small distance should be measured).
- In literature, Tissot indicatrices are used to graphically represent scale factors in different directions and at different positions in a projected map.
- For maps with a conformal projection, the Scale Factor is independent of the direction of measurement.
- For other types of projections, the Scale Factor is generally not the same when measuring for instance in North-South direction or in East-West direction, etc.
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Meridian Convergence:
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The difference between the geographic North and the XY-North of the map, calculated as Ellipsoidal Azimuth - Azimuth on map (degrees).
- The Meridian Convergence is positive when the XY-North points more to the East than the geographic North.
- The Meridian Convergence is calculated for the starting position.
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LatLon-coordinates, Ellipsoidal or Spherical Distance, and Ellipsoidal or Spherical Azimuth are shown when the geographic coordinates are available, i.e.:
- when the coordinate system has a projection (coordinate system of type Projection),
- when the coordinate system is system coordinate system LatLon or LatLonWGS84, or
- when the coordinate system is of type LatLon.
Spherical Distance and Spherical Azimuth are calculated instead of Ellipsoidal Distance and Ellipsoidal Azimuth when geographic coordinates are available, and:
- when an ellipsoid is available, but when the distance is larger than 800 km, or
- when no ellipsoid is available, for instance:
- when the coordinate system has a projection (coordinate system of type Projection), but when no ellipsoid or datum has been selected,
- when the coordinate system is system coordinate system LatLon.
Scale Factor and Meridian Convergence are shown:
- when the coordinate system has a projection (coordinate system of type Projection),
Mind: Results are only reliable when the distance between the start position and the end position is not too large.
See also:
Map window : functionality
ILWIS objects : coordinate system