====== Measurements via triangulation using 2 images ======

This page describes the concept and how to measure via triangulation using two images. \\
More information related to mobile mapping measurements on Orbit Desktop and Orbit WebClient, see : 
  * Desktop : [[112:mobile_mapping:desktop:core:use:measure|]]
  * WebClient : [[112:mobile_mapping:publisher:client:panorama_component_measure|]]

{{ mobile_mapping:desktop:core:use:measure_triangulation_v2.png?nolink&900 |}}

Don't flag the check box “Use Point Cloud” on the Measure Window.

==== Principle of Triangulation ====

{{  mobile_mapping:desktop:core:use:measure_triangulation.png?350&nolink|}}

A 3D coordinate measured via triangulation 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 triangulation requires to measure the corresponding position (pixel) in two views. \\ 
More information about triangulation measurements : [[http://en.wikipedia.org/wiki/Triangulation|Wikipedia Triangulation]].

==== How to measure ====

=== Open 2 Views ===

To complete the triangulation 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.  

=== 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 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.

==== Triangulation error ====

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

==== Visualize triangulation measure rays ====

On doing a measurement based on triangulation it is possible and advised to visualize the triangulation measure lines aka epipolar lines. Orbit displays these as green lines. \\
Triangulation lines are the representation of the measurement as done in one view in the other opened views. These lines simplify the completion of an accurate triangulation measurement.

In Orbit Desktop, triangulation measure lines can be made via the Mobile Mapping preferences. In the Orbit WebClient these lines are visible at all time when performing a triangulation measurement. 

{{orbitgis:how_to_find.png?25&nolink |}} [[112:orbit_desktop:workspace:toolbar|Main Toolbar]] > [[112:orbit_desktop:preferences:index|]] > Mobile Mapping