GIS & Remote Sensing

Parallax for Height Measurement using Aerial PhotographyParallax ConceptPhotogrammetry is capable of measuring elevation of earth surface. Aerial photographs/stereo pair satellite images can be used to measure elevation differences through the use of parallax method. New launched satellite are providing stereo pair satellite images images such worldview-2 etc. Parallax can be defined as the apparent displacement of a point due to a change in view of the point. One of the parallax example for human eye can be explained by a simple exercise of childhood. i.e. hold finger in front of eyes and look at finger, where it is relative to a wall in the background with your right eye. Then look at it with your left eye and its appearance relative to the wall has changed. This relative change in appearance is due to parallax. It took place due to change in view point for the finger. Parallax of Aerial PhotographsParallax can be described by below figure
In this figure point P & Q on surface are c…

In this article discuss about displacement in aerial photograph occurred due to relief or height of object on ground. Relief/Height/Elevation is the most significant source of image displacement. When point/object on the photograph is displaced from its datum position because of its elevation, a displacement take place so this displacement is called Relief displacement or displacement due to height of object. This relief displacement process can also help to calculate the height of object. This relief displacement is explained by below figure. In this Figure, ‘O’ is the camera station. ‘NA' is a flat plain on which stands a tower ‘AB’ with its base at ‘B’.The image of ‘B’ on the truly vertical positive photographic plane is ‘b’. This is the correct planimetric position (orthogonal) of the image of the tower ‘AB’.
Top ‘A’ is imaged at `a'.The image of ‘A’ is thus displaced from its correct planimetric position ‘b’, as `A' is vertically above `B', on the photograph. This sh…

Scale of Aerial PhotographsScale is the relationship between distance on a map or photo and the actual ground distance.Scale is one of the important parameter for aerial photographs/Satellite Images. It is calculated before aerial photography, as number of photographs required to map for specific area calculated based on scale defined for the aerial photography.
There are three method defined for scale measurement for aerial photography. i)By establishing the relation of photo to ground:If the distance between the same two points on the photo as well as on the ground can be measured, R.F. can be set up:
Scale= photo distance/ground distance
Example-
Q1. Distance between two points AB on ground is 1320 ft, distance on map is 3.77 inch. Find out the scale of photo at average ground elevation of this line.
Ans. For this distance on map and distance on ground given in the question therefore below formula will use to solve this question
Scale= Photo Distance/Ground Distance
First bring both dista…