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# Measurements using Forward Intersection

This page describes the concept and how to measure using images only.

More information related to mobile mapping measurements on Orbit Desktop and Orbit WebClient, see :

- Desktop : 3D Measurements
- WebClient : Measure Sidebar

## Principle of Forward Intersection

A 3D coordinate measured via Forward Intersection is the result of the intersection of two 3D measure lines. If there is no 3D intersection, Orbit will use their XY 2D intersection. The Z coordinate will be the average of both Z values at the given XY coordinate. In other words the XYZ coordinate will in the middle of the segment defined by the intersections of both 3D measure lines with the normal of their 2D intersection.

Consequently a coordinate measurement via Forward Intersection requires to measure the corresponding position (pixel) in two views.

More information about Forward Intersection measurements : Wikipedia Triangulation.

## How to measure

#### Open 2 Views

To complete the Forward Intersection measurement correctly both used views cannot not be closed or replaced to a new image position on doing the measurement. It is required to open first both views prior to starting the measurement.

#### Measure perpendicular

To avoid unexpected measure results due to a minor user click error, do measure as perpendicular as possible. The less the measure angle differs from 90 degrees, the smaller the impact can be of a minor operator measure click error on the absolute measure result coordinate.

The smaller the XY angle between both measure lines, the bigger the possible absolute measure fault of a single pixel user click mistake.

#### Sequence of measured coordinates

One measurement can consists out of multiple coordinates. The sequences of measured coordinates in both views must be exactly the same.

This can be either by (a) completing the entire measurement in one view first and repeat the same measurement in a second view or (b) measure coordinate by coordinate in both views.

### Measure Error

The displayed error is the highest individual error of all measured coordinates. A single coordinate error is the average of the shortest distance Z between both 3D straight measure lines and the calculated 3D measure result coordinate.

## Visualize triangulation measure rays

On doing a measurement based on Forward Intersection, the epipolar lines, representing the measurement as done in the first view are displayed as green lines at all time. These epipolar lines simplify the completion of an accurate triangulation measurement.