AT Points Measurement is the most vital process in 3D Aerial Photogrammetry aero-triangulation. This aero triangulation accuracy depends upon the aerial photogrammetry software used. So choosing of the perfect aerial photogrammetry software is also a major factor. The result of an aerial triangulation depends
first of all on the quality of the measurements in the aerial
triangulation process, and also on the point determination.
A frame can have maximum of nine pass points and six tie points. This is because minimum three pass points should be transferred to the next frame and three tie points transferred to the adjacent strip. The common points from the previous frame are always measured first, and the new points will after they have been measured, also be transferred to the next frame and to the adjacent strips and cross strips. At the same time the operator can measure ground control points which can appears on the screen, some ground control points are not paneled, but instead identifiable natural details have been measured, sketches will show the location of such natural details.
A frame can have maximum of nine pass points and six tie points. This is because minimum three pass points should be transferred to the next frame and three tie points transferred to the adjacent strip. The common points from the previous frame are always measured first, and the new points will after they have been measured, also be transferred to the next frame and to the adjacent strips and cross strips. At the same time the operator can measure ground control points which can appears on the screen, some ground control points are not paneled, but instead identifiable natural details have been measured, sketches will show the location of such natural details.
General guidelines of how to select and measure points:
Category I:
Points are clear spots on the ground. The point should be on flat and hard surface, away from large objects trees and shades.
Category II:
Gray or black symmetrical details, such as flat concrete pads with height reference normally on the ground. The point should be on flat and open surface, away from large objects, trees and shades.
Category III:
When no other details are available identifiable patterns (natural or manmade) white or black spots have to be measured. Other points may be footpath crossing, edge of terrain or other symmetrical details.
Category IV:
Details like trees must be used with almost care. Keep in mind that the detail also must appear clearly in image from different angles (from adjacent strip).
Details in motion such as boats on the river, or at sea, cars on the road must never be used.
The chosen details must always be visible in the next photo, check this before measuring current model points.
The chosen details in must cover also appear in the neighboring strip (top bottom).
Sea level points in 3D Aerial Photogrammetry by using aero-triangulation software :
If the model covers sea and coastal areas, one or two sea level points must be measured.
If such models are in the beginning or at the end of photo strip, four sea level points should be measured.
Select sea level points on the coastline, and try to find an area with calm sea, without big waves. For model covering islands with no overlapping connection to other land areas, measure 3-5 sea level points.
Follow next post about the importance of tiepoint in 3D Aerial Photogrammetry by using aero-triangulation software.
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