Pend Photo

8/11/2019 Pend Photo

1/45

Introduction toPhotogrammetry


2/45

What is Photogrammetry?

The science, art and technology ofobtaining reliable measurements, maps,

DEMs and other derived products fromphotographs emphasis on quantitative measurements

length

area height

volume


3/45

Background ofPhotogrammetry

1856 - Toumachon ascends in a balloon (nearParis) and takes the first aerial photo

Aerial photos are interpreted for mappingpurposes

has become a profession some features are clearly visible, while others are

more difficult to recognize


4/45

Background ofPhotogrammetry

Interpretation is based on extractinginformation from shape, size, patterns,

shadow, grey tones, colour, texture andthrough context and comparison withcontiguous areas

black and white (panchromatic)colour (Infrared)


5/45

Advantages of AerialPhotographs

Synoptic views of large areas

stop action capabilities

permanent storage

allows spatial relationships to be

determined which are not possible fromthe ground

cost effective


6/45

OrderBasics

Tone/Colour- distinguishable variationsin shades of black to white or colour

can distinguish many more colours thanshades of grey

Resolution- ability of the entire system

to create a sharply defined image may be discussed in terms of camera lens,

ground resolution


7/45

OrderGeometric Arrangement

Size- can be used for judging thesignificance of objects and features

both relative and absolute sizes important

Shape- aids in identification

man made - tend to have straight edges natural - tend to have irregular shapes


8/45

OrderSpatial Arrangement

Texture- frequency of change andarrangement of tones visual inspection of smoothness or

roughness

e.g. Water typically smooth, grass ismedium texture, and forest is rough

there are always exceptions

Pattern- spatial arrangement of objects

grid network of street, drainage patterns


9/45

OrderLocational or Postional

Site - how objects are arranged withrespect to one another or to terrain

features aspect, topography, geology, soil,

vegetation and cultural features

Association - some objects arecommonly identified with other featuresand tend to confirm the existence ofanother

helpful for manmade installations


10/45

OrderInterpreted

Height- provides detail about many featuresand is useful for analytical studies

height of trees depth of an excavation

Shadow- may enhance or inhibitinterpretation

tree identification can be enhanced by theshadows that are recorded

geologist prefer a low sun angle


11/45

Aids and Techniques forViewing

collateral material

image analysis keys

field verification

handling of imagery

stereoscopic viewing

use of multiple images

convergence of evidence


12/45

How Aerial Photos areCollected

A study area is identified and the airplaneis sent to record a series of successive

photosflight lines - are the straight lines ofplanned flight

nadir line - actual path of the plane tracedon the ground

end lap - over lap between successivephotos along the flight line ensures complete coverage


13/45

Flight Lines


14/45


15/45

Example of Vertical Air Photo


16/45

Low and High Oblique


17/45

High Oblique Aerial Photo

Petaluma River, San Francisco


18/45

x w u rPhoto


19/45

Advantages of Vertical OverOblique Aerial Photographs

Uniform scale

directions (bearing or azimuth) can be

performed in the same manner as a mapeasier to interpret

tall buildings less likely to mask other objects

minimum of mathematical correction req.stereoscopic study is more effective


20/45

Advantages of Oblique OverVertical Aerial Photographs

Covers more ground at the same altitude andfocal length

may be more effective if clouds are presenthave a more natural view because silhouettesare visible

objects not visible on vertical may be visiblefeature elevations are more accurate

may use inexpensive cameras


21/45

Photographic CoordinateSystem

x-y axis defined by connecting lines ofopposite fiducial marks

the point where the x and y axes cross isthe origin

x-axis assigned to fiducial axis most

coincident with the line of flightprogresses in a positive fashion from theorigin in the direction of flight

negative in opposite direction


22/45

Photographic CoordinateSystem

y-axis is perpendicular to the x-axis

intersection of axes is photo-coordinate

originorigin coincides with the principle point(ideally)


23/45

Photo Centres

Most aerial photos are not perfectly vertical

three photo centres

principle point nadir

isocentre

each is important in terms of different types of

geometric distortions

if perfectly vertical, all three centres wouldcoincide at one point (principle point)


24/45

Photo Centres


25/45

Principle Point

Geometric centre of the the photo

occurs at the intersection between theprojection of the optical axis and theground

defined by the intersection of lines

drawn between opposite fiducial marks

defines the origin of the x and y axes inthe photo coordinate system


26/45

Nadir

also known as the vertical or plumb point

intersection between the plumb line directly

beneath the camera centre and the ground atthe time of exposure

unlike the principle point, there are no markson the photo to locate the nadir

locating nadir requires stereoscopic plottingtechniques and expensive instruments andground control information


27/45

Nadir

Relief displacement is radial from nadir so tallbuildings can be used as an indicator of nadir

by projecting lines along the displacementedges of those features

if photo not perfectly vertical, the nadir andprinciple point have different positions

nadir is always on the down side of the tiltedphotograph


28/45

Isocentre

The point halfway between the principlepoint and the nadir on a line joining

those two pointsis the focus of tilt displacement

on a truly vertical photo, there would be

no tilt displacement only reliefdisplacement and radial displacement


29/45

Radial Displacement

On vertical photos, is the apparent leaningout or displacement of the top of any objecthaving height in relation to its base

direction of displacement is radial from theprinciple point on a true vertical photo orfrom the isocentre on a vertical photograph

distorted by tilt of the platform


30/45

Relief Displacement

Displacement due to variation in terrainelevation

an increase in elevation causes features in thephoto to be displaced radially outward from theprinciple point and nadir

increases as distance from pp increases

increases as elevation increases

decreases with increases of flying height

no relief displacement at principle point


31/45

Relief Displacement


32/45

Tilt Displacement

Displacement due to tilt of the platformrelative to the ground

degree of displacement increases awayfrom the isocentre


33/45

Tilt Displacement


34/45

Scale

States that one unit of distance on thephoto represents a specific number ofunits on the ground

unit equivalent scale e.g. 1mm on photo represents 25m on

ground

representative fraction e.g. 1:50 000 (unitless)


35/45

Scale

Large vs small scale: Large scale = small area with lots of detail

Small scale = larger area with less detailMethod for determining scale measure distance on photo and compare

to actual ground distance

must convert to same units photo distance/ground distance


36/45

Example of Large Scale

This is a 1:5000 air photo


37/45

Example of Small Scale Photo

This is a 1:50000

air photo


38/45

Scale Variation

Within the same photo, there isgeometric distortion

all points on a map are depicted in theirtrue planimetric relationships

all points on a photo are not depicted in

their true planimetric relationshipphoto results from projecting convergingrays through a common point in a camera

lens


39/45

Variables


40/45

Calculating Scale

S = d/D

where

S = photo scale

d = photo distance

D = ground distance


41/45

Example Calculation

If you are given an aerial photographwith a scale of 1:5000 and a building is

measured with a width of 15.3 mm,what would the width of the building beif measured on the ground?

S = d/DAnswer: 76.5 m


42/45


43/45

Calculating Scale

S = f/H

where

S = photo scale (RF)f = camera focal length

H = flying height above terrain

H = H - hh = terrain elevation


44/45

Example Calculation

If the scale of a photo is 1:15000 and avertical photograph is taken from a

flying height of 2780m above mean sealevel, what is the focal length of thecamera, if the terrain is flat and locatedat an elevation of 500m?

S = f/H

Answer: 152 mm


45/45

Ground Coverage

Function of:

camera format size

focal length

flying height above terrain

format size is the actual size of the imagerecorded on film

larger format = larger ground coverageshorter focal length = larger ground cov.

Lower flying height = less ground coverage

Documents

Pend Photo