ESPM 172 Photogrammetry & Remote Sensing
Lab #5 Scale Measurements
This work is due at the beginning of the next lab period.
Remember to make all measurements on the photographs and maps in mm; ground distances have to be converted to meters if measured in feet.
For Part A use photo #4 of the campus
Part A Estimation of point scales or scales based on two points.
1. The photographs we have were taken from a flight height of 6000' above the sea level with a focal length of 6". Select 3 points on photo 4 that can be located on the USGS 7 1/2 minute quad (Oakland West) topographic map. Mark and label the three points on an acetate. Read the elevation of the three points from the topo map and determine the point scales for the three points.
2. Select two line segments from photo 4 and mark and label them on the acetate. Measure the photo distance for each line segment. Determine the ground distances for each line segments using the two campus maps. Calculate two scales for each line segment. Which estimate is the best? Why? What is wrong (if anything) with the other estimate?
3. Using photo 4, calculate the photo scale using the length of the football field (the ground distance is 120 yards see diagram on the board).
Part B. Average Scale
1. Compute an average scale for each photo (1-3, 1-4) using at least 2 measurements on each photo. Record each measurement and then the average for each photo. Use your knowledge of photo scale to choose the best places to estimate scale. Explain your choices. Use the topo map to obtain the corresponding ground distances needed in your calculations. Make sure to show all calculations and measured distances. Summarize your results in a table with 2 measurements of scale for each photo, the average scale for each photo, and the average scale for all the photos. Mark on the acetate for photo 4 the line segments used for average scale calculation.
2. Using the enlarged campus photos (1-3, 1-4) calculate the average photo scale using at either known ground distances (such football fields, tennis courts, etc.) or distances measured from maps.
3. Measure at least 2 ground distances from the large scale campus map. These can be measurements such as lengths, or widths of buildings, or the distance between two points that can be distinctly identified on photos. After collecting these data, determine the average photo scale for photo 4. Compare this scale estimation method with that used in question 3 in Part A. Which method is better and why is it better?
4. Briefly summarize how accurate the methods used in this lab are and how difficult it is to get the information needed for each particular method.