Scalling of Photograph (GIS AND RS) -1.pptx

Scalling of Photograph (GIS AND RS) -1.pptx

1. Measurement Scale

2. A photograph or a map is necessarily smaller than the actual ground areas it represents. To be able to relate or establish a relationship between the photographed or mapped areas T heir corresponding actual areas on the ground, the photograph -or the map- must state the ratio or proportion between the entities it represents and their corresponding entities on the ground Scale is one of the most important information for the usage of an aerial photograph or a map.

3. Quantitative measurements and interpretation of features on a photograph are highly dependent upon this information. Scale is what determines the relationship between the objects imaged on a photograph and their corresponding in the real world (i.e., the ground) The absence of scale makes it impossible to relate the size of or the distance between objects on a photograph to their actual sizes or distances on the ground. 

4. The ratio of the distance measured between any two points on the photograph (or a map) to the distance between the same two points on the ground is called Scale of Photograph In figure ‘ the photo scale is expressed as   oa /OA  (or   ob /OB  or   ab /AB ). This statement means that one unit of distance on a photograph (or a map) represents a specific number of units of actual distance on the ground.

5. A scale of 1:20000 simply means that a distance of 1 mm measured on the photograph (or a map) represents 20000 mm (20 meters) on the ground, or 1 cm on the photograph represents 20000 cm (200 m) on the ground, or 1 inch on the photograph represents 20000 inches (or 1667 feet) on the ground.  Scale determination requires both the ground and the photo distances to be measured along the horizontal plane for accurate representation of the photo scale. Unfortunately , unlike maps, most photographs are subject to distortions  

6. Representing Scale Representative fraction  (or ratio ) The fraction of a distance measured between two points on a photograph to the distance measured between the same two points on the ground. It can be expressed as 1/20000 or as 1:20000. For printing convenience, the latter form is the most commonly used. Notation for  RF  can also be written as  PS   (photo scale ). Unit equivalents , Also called   equivalent scale , The equivalence of a distance measured between two points in photographic units to the distance between the same two points in ground units . For example, a PS of 1:20000 would be expressed as 1 mm = 20 m (or 1 cm = 200 m or 1 inch = 258 ft ), meaning that a distance of 1 mm on a photograph is equivalent to 20 m on the ground (or 1 cm is equivalent to 200 m on the ground).

7. To convert from the representative fraction ( RF ) to the unit equivalents ( UE ), W e simply multiply both the numerator and the denominator of the former by unity so that all units cancel to obtain the unit equivalents desired . For example, if we desire to obtain unit equivalents of centimeters per kilometer for a  RF  of 1:20000 , we proceed as follows :

8. written as 5 cm = 1 km and reads 5 cm measured on the photograph represents 1 kilometer on the ground.

9. Photoscale =Photo distance/ground distance 2:5000 2/50000*1000m/1km=1m/25km 1m=25km

10. Photo scale reciprocal  ( PSR ) Simply the inverse of the representative fraction . For example, an  RF  of 1:20000 would correspond to a  PSR  of 20000 . Among the different expressions of the scale, the scale reciprocal (i.e.,  PSR  or  MSR ) may be the most convenient to work with in constructing equations . However, its interpretation may result in confusion, because large values of  PSR  -or  MSP - represent small scales and vice versa. For example, 24000 scale photographs are larger scale than 36000 scale photographs

11. Methods of determining scale of a vertical photograph Scale from photo and ground measurements The basic and most straightforward technique of expressing scale is the one that uses the ratio of the distance between two points on a photograph ( PD ) to the distance between the same two points on the ground ( GD ). PS = PD/GD MS=MD/GD Scale=Focal length/Flight Height PS  is the photo scale,  MS  is the map scale,  PD  is the photo distance measured between two well identified points on the photograph,  MD  is the map distance measured between two points on the map, and  GD  is the ground distance between the same two points on the photograph (or on the map), expressed in the same units.    

12. QUESTION Assume that the distance between two points was measured to be 83.33 mm on a vertical photograph and 125.00 mm on a map. If the surveying ground distance between the same two points is 3000 m, what are the scales and the scale reciprocals of the photograph and the map ? Using the information in example ABOVE express the photo scale and the map scale in unit equivalents.  MS=125mm/3000000mm

13. A photograph is annotated with information T he Date and time of acquisition , The project code, T he serial identification of the photograph (i.e., line number and exposure number), T he focal length, The flying altitude of the aircraft  This nomenclature is printed on the west side for photographs flown east-west and on the north side for photographs flown north-south

14. Scale from focal length and flying height on Flat elevation These parameters (Mention on Photograph) are present, the photo scale may be determined using the same geometry of a vertical aerial photograph  :Triangle   Lop  and  LOP ,

15. PS= f/H-h PS=Focal Length/[Flight Altitude – Terr.Elevation PS  is the photo scale,  f=   is the focal length of the camera used to take the photograph   H-h=  T he flying altitude of the aircraft above the ground, which may be the average elevation of the entire project area, the average elevation of the photograph -or between two points on the photograph, or the elevation of a single point on the ground.

16. Aerial photograph is not a controlled map and that a photographic scale determined in this way is a mean or averaged scale In a vertical aerial photograph the displacement of images is in a radial direction from the centre point of the photograph. This displacement is termed the radial displacement due to relief and represents an error in map positioning

17. One aerial photograph the amount of radial displacement, m, of the top of an object from its base, can be determined by the relation m =r h/H r = radial distance on the photograph from the centre point to the top of the image displaced h = height of the object displaced, and H = the flight height

18. Rearranging equation yields a convenient expression for estimating the height of an object on a photograph by measuring its radial displacement: h =m/r x H

19. Determining the height of an object on a single vertical aerial photograph is by measuring shadow length Object Height/Shadow length If surface is flat Object height is known

20. If object height is not known then Height can be calculated by using shadow length if the angle of inclination of the sun's rays is known Angle can bedetermined from tables of sun angle by latitude and time of the year h = Ltanß L = the actual length of the shadow on the ground ß = the angle of inclination of the sun

21. QUESTION   A photograph was taken from 3500 m above MSL with a 152.4-mm focal length camera as printed on the photograph. If the average ground elevation of the area covered by the photograph is 830 m above MSL, what is the scale of the photograph ? PS= f/H-h ANSWER?

22. Scale from photo and map measurements The information on the photograph and the ground distance between two points are not available, the photo scale can still be determined if a map of the area is available . PS= PD/GD OR PSR=1/PD/GD= GD= PD x PSR MS=MD/GD OR MSR=1/MD/GD= GD=MD x MSR BY examining bolded equation PD x 1/PS = MD x MSR PS=PD/MD x MSR PD  and  MD  are distances measured between the same 2 points that are well identified on the photograph and the map

23. QUESTION The distance between the same two points,  A  and  B , were measured to be 6.05 cm on a photograph and 4.25 cm on a 7.5-minute USGS quadrangle (MSR = 24,000), then find the scale of the photograph ( PS ) PS=PD/MD x MSR = ANSWER

24. This method is the most useful and the most practical form of determining a photo scale, simply because of easily obtainable values from a map Many countries and many regions are not entirely covered with maps or ground surveying distances. If a situation arises where ground surveying distances, a map of the area, and the information on a photograph are not available, a photo scale can still be determined if another photograph of the same area and with known scale is available.    

25. Scale from an existing aerial photograph When the scale of a second photograph ( PS 2 ) is known, our photo scale ( PS 1 ) may be determined in the same manner as it was done using a map . By measuring the distances between the same two points on photo 1 ( PD 1 ) and photo 2 ( PD 2 ), the unknown scale of photo 1 ( PS 1 ) may be determined from the known scale of photo 2 ( PS 2 ) as PS 1 =PD 1 /PD 2 x PSR 2 PS 1  is the scale to be determined for the photograph considered (photo 1),  PD 1  is the distance measured between two well identified points on photo 1,  PD 2  is the distance measured between the same two points on the known-scale photograph (photo 2), and  PSR 2  is the scale reciprocal of photo 2.

26. QUESTION Two points were clearly identified on an aerial photograph and on a 1:24000-scale USGS topographic map of the same area. The distance on the photograph between the two points was measured to be 8.25 cm and the distance on the map between the same points was measured to be 5.72 cm. Find the scale of the photograph . ANSWER

27. Scale from an existing aerial photograph This situation requires that either the scale or the focal length and the flying height above the ground of a second photograph are known. When the scale of a second photograph ( PS 2 ) is known, our photo scale ( PS 1 ) may be determined in the same manner as it was done using a map . By measuring the distances between the same two points on photo 1 ( PD 1 ) and photo 2 ( PD 2 ), the unknown scale of photo 1 ( PS 1 ) may be determined from the known scale of photo 2 ( PS 2 ) as: 

28. PS 1 =PD 1 /PD 2 x PSR 2 PS 1  is the scale to be determined for the photograph considered (photo 1),  PD 1  is the distance measured between two well identified points on photo 1,  PD 2  is the distance measured between the same two points on the known-scale photograph (photo 2), and  PSR 2  is the scale reciprocal of photo 2.

29. If the scale of photo 2 is not known but the focal length and the flying height above the ground are available, then, the scale of photo 1 may be determined as follows : PS 1 =PD 1 x f 2 /PD 2 x (H- h 2 ) f 2  is the focal length of the camera used to take photo 2 H-h 2  is the flying height of the aircraft above the ground of photo 2.

30. Procedure of determining scale for a single photograph If the information usually printed on an aerial photograph is missing but a map of the area or ground measurements are available , the average scale of a single photograph may be determined as follows:  Select two points, preferably at the average ground elevation of the photograph, and approximately equidistant from and opposite the photo center. The two points must accurately be located on a map -or on the ground if a map is not available. Accurately (to nearest 0.1 mm) measure the distance between the two points on the photograph and the map -or determine the ground horizontal distance (to nearest 1 m) if a map is not available. Compute the photo scale using equation on slide 20 if map measurements are available or using equation on Slide 12 if ground measurements are available.

31. Question A 305-mm focal length was used to take aerial photographs from 4000 m above MSL. Using a topographic map, the average elevations of the flight lines were found to be 700 m for line 1, 580 m for line 2, 650 m for line 3, and 750 for line 4. Find the average scale of the project.

32. Question Now, suppose that neither a map nor the information on the photograph are available, but the ground distance between two points well identified points is known to be 1320 m and the distance measured on the photograph between the same two points is measured to be 7.86 cm. Find the scale of the photograph.