Geodesy and distance calculation

Distance Calculations

Geocap can easily calculate the shortest distance between two points following the earth's curvature. It can also calculate horizontal and vertical distances measured in 3D. All this is done with the Calculate earth distance menu. The menu is opened from Tools > Distance Calculations.

Menu for correction distance calculation

How to calculate a distance

1. When opening the menu you will be asked if you want to switch to 2D mode. Click Yes if you want to measure a distance following the earth's curvature. If you want to measure horizontal or vertical distances, click No to keep the graphics view in 3D mode.

2. Set the datum and coordinate system of the data in the display window.

3 Select which unit you want for distances. Default unit is in meters.

4. Select which distance calculation method you want to use.

• Flat earth calculates the distance between the two points as a straight line. Flat earth will automatically use Earth centered sphere if the selected coordinate system is geodetic.
• Earth centered sphere approximates the earth as a sphere with a constant radius.
• Radius adapted sphere approximates the earth as a sphere, but uses the radius of a sphere with the same curvature as the ellipsoid used by the selected datum, at the midpoint between two points.
• Ellipse approximation calculates the distance along the ellipsoid using an approximation algorithm.
• Vincenty calculation calculates the geodesic distances using an accurate ellipsoidal model of the earth.

5.a) To measure a distance following the earth's curvature (2D):

1. Click (left mouse button) in the graphics window to set the start point. The menu will show the coordinates of the point.
2. Click (left mouse button) in the graphics window to set the end point. The distance between the two points is shown by a line, and the calculated distance is shown at the bottom of the menu (distance).

5.b) To measure a distance between two points (2D/3D)

1. Point your cursor on the first point and press j on the keyboard to snap to that point.
2. Point your cursor on the second point and press j on the keyboard yo snap to that point.

5.b) To measure a vertical and/or horizontal distance (3D):

1. Position the mouse cursor on the start point and press Y on the keyboard. The menu will show the coordinates and depth of the point.
2. Position the mouse cursor on the end point and press Y on the keyboard. The distance between the two points is shown by a line, and the calculated distances (vertical, horizontal and distance) is shown at the bottom of the menu .

6. The distance is listed in the bottom of the distance calculation menu.

The distances and point coordinates are also listed in the message window (Pulldown: Windows > Message Window) like this:

x:-993260.1  y:-3165685  z:-513.3781  lat:-27.49463°  lon:-8.922608°  distance:118.6007M
x:-789037.2  y:-3270633  z:-513.3781  lat:-28.33208°  lon:-7.088041°  distance:109.6335M
x:-1135082   y:-3245105  z:-513.3781  lat:-28.12898°  lon:-10.19661°  distance:165.1901M

Calculate distances between the points in a dataset

For Geocap Shelf users the distance to previous point is calculated by several operations, and may be found in Table View in a Field Data column. It is possible to create this type of Field data columns by using a command called Calculate line distance... found under Shared commands > Analysis. The results will be stored in Table View after running this command.