Aerial Triangulation or Aero-Triangulation (Chap 17)
This is the process of extending or densifying the available ground
control which will be needed for orientation of individual models.
Each stereo model needs 3 horizontal control points and 4
elevation control points for absolute orientation. A large mapping
projects may consist of hundreds of models. If all these points are
to be surveyed in the field, then the cost of the project may be too
high.
Aerial triangulation may also be used for getting accurate
coordinates for discrete points such as section corners.
Aerial triangulation will also help detect any blunders in ground
control. Usually the most costly part of photogrammetric mapping
is the ground control. Number of field surveyed ground control
should be kept to a minimum.
The process is not limited to aerial photogrammetry, and therefore,
may be called ‘photo-triangulation’.
Aerial triangulation cab be accomplished
• Using a stereoplotter (analog)
• Semi-analytical
• Analytical
1
Analog method needs a suitable stereoplotter to join all the models
(bridging) along a flight line (strip triangulation). Strips are
connected together(block triangulation) using common points (tie
points) on the side laps.
In semi-analytical method, model coordinates of points are
measured on a stereoplotter and the block is adjusted
mathematically.
Analytical method uses measured photo coordinates in collinearity
equations. The ground coordinates of pass points and any new
points as well as elements of exterior orientation for all the photos
are determined in the process.
Analytical method can be used to triangulate along strips and the
block is adjusted same way as in semi-analytical method.
Measured photo coordinates need to be corrected for all image
displacements discussed in Chap 4.
An analytical method called ‘bundle adjustment’ is used to
triangulate an entire block of photos simultaneously. This method
can provide ground coordinates with geodetic accuracy depending
on the accuracy of existing control (photo-geodesy)
2
If analytical aerial triangulation is planned, field points can be
paneled before the photo flight in order to increase the accuracy of
measuring photo coordinates.
Ground Control for Aerial Photogrammetry (Chap 16)
Ground control points are points which have been surveyed in the
field to determine their exact locations in terms of some reference
coordinates. These points should also be easily identifiable and
well defined in all photographs they appear. Best points for
horizontal control are those which can be identified clearly on the
photos.
Examples of good points for horizontal control are fence corners,
base of power poles, manhole covers etc. Same points can serve
as both horizontal and vertical control or they may be different
points.
Vertical control points should be at locations where floating mark
can be placed in vertical direction accurately, but their horizontal
position need not be sharp. Examples of vertical control are road
or sidewalk intersections, isolated rocks with small crowns etc.
Points in large flat areas are not suitable as vertical control.
3
Ground control is used for rectification, orientation of stereo-pairs,
and in aerial triangulation. In order to achieve optimum results,
they must appear in certain strategic locations of the overlap area
or project area. For this reason, ground control is surveyed after
the photos have been acquired, but for high precision work such
as analytical aerial triangulation, the points are marked and targets
are placed before flying.
Ground control must always have a much higher accuracy than
that of maps that they will help make. Currently, the fastest and
most economical methods of establishing ground control (photo
control) is by GPS
The number of ground control points required may be reduced by
having GPS receivers on board to determine the coordinates of
exposure station and attitude of the aircraft directly.
Project Planning (Chap. 18)
Success and cost containment of any of a photogrammetric
mapping project depends on good planning which requires both
knowledge and experience.
Planning usually includes
• flight or mission planning
• ground control considerations
• cost estimation and scheduling
4
Flight planning depends on
• purpose of photography
• project area size and photo scale or final map scale
• topography and vegetation in the project area
• existing ground control
• season of the year
For mapping, the photo scale depends on the method of map
compilation. If conventional stereo-plotters are used, limitations of
the plotter will control photo scale
Plotter limitations may include one or more of the following
• camera focal lengths that can be accommodated
• enlargement ratio from photo scale to map scale
• c-factor which affects the accuracy of elevations
Analytical plotters generally do not have any limitation such as
above but achievable accuracy both in planimetric positions and
elevations depends on the photo scale which in turn depends on
the flying height
Photo scale also limits the size of smallest recognizable ground
objects or features
5
If softcopy techniques are to be used for map compilation,
scanning resolution too affects the size of smallest objects that
could be mapped
Flight plan will include
• flying height
• air-base and spacing between flight lines
• Information such as camera, film and tolerances on tilt,
crab, and drift which result in loss of overlap
• a flight map
Entire flight plan could be downloaded into an on-board computer
which will guide the flight
Examples:
