A vertical photograph was taken from a height of 3200m above MSL with a camera of focal length 120 mm. it contains two points a and b corresponding to ground points A and B. calculate the horizontal length AB.
| Photo point | Elevation above MSL | Photo coordinates | |
| x(mm) | y(mm) | ||
| a | 640m | +19.50 | -14.60 |
| b | 780m | +26.70 | +10.80 |
Enter numerical value using the virtual keypad. Round off where necessary.