engi 3703 surveying and geomatics - faculty of …sitotaw/site/fall2007_files/lecture20.pdf · engi...

ENGI 3703Surveying and Geomatics

TopicInstructor: Prof. Ken Snelgrove

Lect 20 - Nov 5/07 Slide 1 of 7Photogrammetry - Basics

Photogrammetry: (Chapter 23) Is defined as the science associated with obtaining reliable information

from photographs. These techniques complement the mapping techniques you have been exploring in

your laboratory sessions. Large amounts of data can be extracted from photographs quickly when

compared to terrestrial measurement. This can result to tremendous cost savings. There are two

general specializations:

i) Metrical - information (distances, elevations, angles) are derived from aerial platforms for the

purpose of topographic mapping. Space based imagery is also being more widely used as resolutions

increase. (see: http://www.geoeye.com and http://www.digitalglobe.com)

ii) Interpretative - classification of landscape features such vegetation type and environment change

from the reflectance of various wavelengths of EM (UV, visible, IR, radar), sizes and shapes of

features. These are typically monitored over large areas so high flying (space based) platforms are

typically used. This is generally referred to as remote sensing.

Quickbird view of Athens Olympic StadiumSt. John’s Harbour Air Photo - 1944




Aerial Cameras: Used to take a large number of high quality

images at a fast rate. Film is typically high resolution of size

9 x 9 in (vs 35 mm (1.4 in) used in home film camera) and focal

length 6 inches (152.4 mm).

Digital photography is used primarily today but aerial

photography dates back approximately 100 years ago.

Parts of a film based aerial photo cameraLarge lens to gather light quickly

(keeps shutter speeds fast)

Aerial camera mounted in airplane

Film Canister Library




National Air Photo Library (http://airphotos.nrcan.gc.ca/) St. John’s - 1995

#1

#1 #1

#1

#1#1

#1

#2

#3

Elements of an Vertical Air Photo:

1) Fiducial Marks printed on film

by the camera to determine image

center (Principal Point)

2) Altimeter reading to determine

flying height

3) Roll and frame number to

locate image in time and space.

Oblique Air Photo of St. John’s Harbour.

How does resolution change?

#1

Note: while air photos may look like maps, they are not! Aircraft pitch and roll, scale distortions due to elevation,

camera lens distortion, etc. all contribute to error. These are removed using perspective geometry.




Flight Lines and Index Maps: at the end of a photo mission,

the aerial survey contractor plots the location of the first,

last, and every fifth photo centre, along with its roll and

frame number, on a National Topographic System (NTS)

map. Photo centres are represented by small circles, and

straight lines are drawn connecting the circles to show

photos on the same flight line.

Overlap: is the amount by which one photograph includes the

area covered by another photograph, and is expressed as a

percentage. The photo survey is designed to acquire 60 per

cent forward overlap (between photos along the same flight

line) and 30 per cent lateral overlap (between photos on

adjacent flight lines).

from: National Air Photo Library web site

Vertical Air Photo Geometry: the exposure station is the location of

the camera lens. Light is gather from the scene and focused onto the

backplane of the camera at the focal length (Lo’). A contact print

positive can be constructed in front of the lens that is an exact 180°

reversal of the negative. Here we consider an x-y coordinate system

centered at on the contact print and intersecting the line Ooo’ defined

by the fiducial marks of the camera. Note: the x-axis is parallel to the

direction of flight.




Vertical Image Scale: (Section 27.6) Scale in mapping is the distance between two points on a map

divided by the actual ground distance between the two points. Since a map is an orthographic projection

map scales are constant. Aerial photograph scales relate the distance on a photo to ground distance.

However, photo scales are not constant and varies from place to place depending on the terrain

elevation.

f - focal length of lens

o - principle point of photo

H - flying height

hx - elevation of x

Problem: AB=CD but ab cdfrom Similar LAB& Lab

Photo Distance

Ground Distance=ab

AB=La

LA

from Similar LOA& Loa

La

LA=

f

H ha

assuming AB is small the scale (SA )

SA =f

H ha

In general points on a photo different scales as

S =f

H h

Savg =f

H havg

photo scale

map scale=

photo distance

map distance

Helpful averaging relations




Ground Coordinates from Photo: (Section 27.7) An arbitrary x-y coordinate system can be transferred

from the photo to the ground by considering the scale of the photo. These must consider the ground

heights hA for point A and hB for point B

from Similar LOAA '& Loa '

oa '

OAA '=

f

H hA=xaXA

we can solve for XA as:

XA =xa H hA( )

f

Similarly,

YA =ya H hA( )

f

engi 3703 surveying and geomatics - faculty of …sitotaw/site/fall2007_files/lecture20.pdf · engi...

Documents