What is photogrammetry?

Photos – light Gramma – to draw Metron – to measure“Photogrammetry is the technique of measuring objects from photographs”

“The art, science and technology of obtaining reliable spatial information about physical objects and the environment through the processes of recording, measuring and interpreting image data.”

Two distinct types of photogrammetry:– Aerial / spaceborne photogrammetry– Close range photogrammetry

Remember this: Photogrammetry is the metric interpretation of image data

Photogrammetry portrayed as systems approach. The input is usually referred to as data acquisition, the “black box" involves photogrammetric procedures and instruments; the output comprises photogrammetric products.

What is photogrammetry?

Topographic photogrammetry

• Used for mapping Earth or planets• Imaging system is based on an

aircraft or spacecraft• Target is the ground surface• Image data is processed to create

new spatial information products

Close range photogrammetry

• Used for industrial measurement• Imaging system is handheld• Target is the object being

measured• Image data is processed to make

precise measurements

What can photogrammetry do?• The simple answer: make accurate 2D and 3D measurements• Information required: images and sensor (camera) parameters

• Close range photogrammetry: 3D only• Topographic photogrammetry: 2D and 3D, depending on specific application

Singlephotograph +

Sensorparameters =

2Dmeasurement

Multiplephotographs +

Sensorparameters =

3Dmeasurement

WHY PHOTOGRAMMETRY

• VERY PRECISE• TIME EFFECTIVE• COST EFFECTIVE• BASED ON WELL ESTABLISHED AND

TESTED ALGORITHMS.• LESS MANUAL EFFORT• MORE GEOGRAPHIC FIDELITY

Cont……

• Corrects all sorts of distortions.• provide a reasonable geometric modeling

alternative when little is known about the geometric nature of the image data.

•  provide an integrated solution for multiple images or photographs simultaneously

• achieve a reasonable accuracy without a great number of GCPs

• create a three-dimensional stereo model or to extract the elevation information

PHOTOGRAMMETRY – Functional Details

Types of photographs

Image RequirementsImage Requirements

• A block should have at least one pair of images (Satellite or Photo) which overlap:

Stereo Pair

Overlap Region

60% Overlap

Overlap Between RunsOverlap Between Runs

30% overlap between runs or swaths

Run #1

Run #2Not essential but can help

Stereo Vision

Left Right

baseline

Matching correlationwindows across scan lines

depth

),(),(

yxd

BfyxZ

Z(x, y) is depth at pixel (x, y)d(x, y) is disparity

Cameras and SensorsCameras and Sensors

Pushbroom Sensors - data is collected along a scan line, each scan line has it's own perspective center.

Perspective Centers

Camera and Sensor TypesCamera and Sensor Types

Cameras– Frame Camera– Digital Camera– Video Camera (Videography)– Non-Metric Camera (35m, Medium and Large

Format Cameras) Pushbroom Sensors

– Generic– Spot– IRS-1C

Stereo aerial photography

Photogrammetry and remote sensing• Photogrammetry → metric exploitation of imagery• Remote sensing → thematic exploitation of imagery

• SPOT (French RS satellite) broke down the barriers in the 1980’s and 1990’s with:– digital linescanner– stereo imaging capability– high quality optics and orbital model

suitable for regional stereo mapping and remote sensing

• Ikonos and Quickbird (launched recently) provide high resolution stereo imagery

suitable for large scale mapping and remote sensing• Next step: digital aerial frame cameras

Digital Cameras  The image plane of a digital camera to be used to record spatial objects contains a two dimensional field of sensors.

CCD sensors (Change Coupled Devices) predominate in digital photogrammetric cameras. Such cameras are known as CCD Cameras

Satellite topographic mapping

• Stereo data can be collected on same orbit, or different orbits(beware of changes)

• Satellite may have to be rotated to point sensor correctly

• Optimum base to height ratio is 0.6 to 1.0

• Atmospheric effects (refraction, optical thickness) become more significant at higher look angles

Different orbits

Same orbit

Stereo satellite imagery

SPOT

• 1 panchromatic, 3 multispectralchannels

• Panchromatic pixel size of 10m

• Multispectral pixel size of 20m

• Good for relief mapping at1:50000

High resolution satellites

Ikonos (SpaceImaging)

• 1m panchromatic and 4m multispectral imagery(NIR, R, G, B)

• 11 bit dynamic range

• Camera specifications not available

Quickbird (Digital Globe)

• 0.61m panchromatic and 2.44m multispectral imagery (NIR, R, G, B)

• 11 bit dynamic range

• Camera specifications available

BRANCHES OF PHOTOGRAMMETRYAnalogue Photogrammetry -

optical or mechanical instruments were used to reconstruct three-dimensional geometry from two overlapping photographs. The main product during this phase was topographic maps.

Analytical Photogrammetry The computer replaces some expensive optical and mechanical components.

The resulting devices were analog/digital hybrids.

Analytical aerotriangulation, analytical plotters, and orthophoto projectors were the main developments during this phase.

Outputs of analytical photogrammetry can be topographic maps, but can also be digital products, such as digital maps and DEMs

Digital Photogrammetry

Digital photogrammetry is applied to digital images that are stored and processed on a computer.

Digital photogrammetry is sometimes called softcopy photogrammetry.

The output products are in digital form, such as digital maps, DEMs, and digital orthophotos saved on computer storage media.

DIGITAL PHOTOGRAMMETRYDIGITAL PHOTOGRAMMETRY

– Single or pairs of digital images are loaded into a computer with image processing capabilities.

– Images may be from satellite or airborne scanners, CCD cameras or are conventional photographs captured by a line scanner.

– Images are either displayed on the screen for operator interpretation, enhanced by image processing or subjected to image correlation in order to form a digital elevation model (DEM) or extract details.

DIGITAL PHOTOGRAMMETRIC DIGITAL PHOTOGRAMMETRIC WORKFLOWWORKFLOW

Creating a 3-D model or map is a straight and linear process that includes several steps-

– Sensor model defenition

– Ground Control Point (GCP) measurement

– Automated tie point collection

– Block bundle adjustment (i.e. Aerial Triangulation)

– Automated DEM extraction

– Ortho-rectification

– 3-D feature collection and attribution

Applications of Photogrammetery

• Topographic mapping

• Creation of value added products:

– Orthoimages

– Digital Elevation Models

– Virtual landscapes

• Nadir imagery is essential for mapping

• Overlap and sidelap is required to give 3D information

Example products: maps

Example products: DTMs

Example products: Virtual landscapes

DTMs

• Sources of terrain models– Stereo photogrammetry– Interferometric radar– Stereo radar– Laser scanning– Digitising maps

Contour digitizing

• Digital terrain model – Digitizing contour maps

• Two step procedure: click on the contours, then grid the data

• Liable to error if the DEM spacing grid is too large

Contour digitizing

Contour map

Radar mapping from space

• Two methods of topographic mapping using synthetic aperture radar (SAR):– Stereo imaging (radargrammetry)– Radar interferometry

• Advantages of Radargrammetry:– 24 hour imaging– Physics of the imaging process is well understood– Radar relief mapping at 1:50000 is possible

• Disadvantages of Radargrammetry :– Radar images have significant geometric distortions– Shadow and layover caused by relief– Radiometric interpretation is difficult

Radar mapping from space

Stereo radar imagery

DEM

Stereo photogrammetry

Airborne laser scanning

• Digital terrain model generation – airborne laser scanning

• Laser pulse is emitted from the sensor – return journey time is measured, giving distance between sensor and target

• Location of the sensor is determined by GPS

• Therefore target can be located

• Significant post processing is required:

– Data thinning

– Gridding

LIDAR data

Horizontal resolution: 2mVertical accuracy: ± 2cm

Modelling building and topological structures

• Two main approaches:– Digital Elevation Models (DEMs) based on

data sampled on a regular grid (lattice)

– Triangular Irregular Networks (TINs) based on irregular sampled data and Delaunay triangulation

DEMs and TINs

DEM with sample points TIN based on same sample points

Advantages/disadvantages

• DEMs:– accept data direct from digital altitude matrices– must be resampled if irregular data used– may miss complex topographic features– may include redundant data in low relief areas– less complex and CPU intensive

• TINs:– accept randomly sampled data without resampling– accept linear features such as contours and breaklines

(ridges and troughs)– accept point features (spot heights and peaks)– vary density of sample points according to terrain

complexity

DEMHill shaded DEM

Aspect Shaded Aspect

Slope Slope draped on DEM

DEM derived Variables

height

slopeaspect

hillshading

plan curvature

Feature Feature extractionextraction

DEM derived Variables

Hydrologic Modeling

• DEMs allows automated modeling of hydrology• Hydrologic modeling is a process that begins with

a DEM• Types of models: flow direction, flow

accumulation, watershed delineation, and flow length

Flow Direction

• Calculates flow direction for every cell in the GRID

• Based on direction of steepest slope in local neighborhood

1

248

16

32 64 128

Flow Accumulation

• Based on flow direction grid

• Creates a grid of accumulated flow to each cell

• Can be used to create a stream network

Flow Accumulation -> Streams

• Use a conditional statement to create a stream network from a flow accumulation grid

• streams =con (flow > 100, 1)

Delineating Watersheds

• Determines the contributing area above a set of cells in a grid

• Needs a flow direction grid and sources

Flowlength

• Calculates distance along a flow path for each cell

• Goes either upstream (to the nearest ridge) or downstream (to the nearest sink or outlet)

• Primary use is to determine the length of the longest flow path in a basin

Visibility Analysis

• Requires:

– DEM

– Viewpoint/s

• What it tells you:

– Line of sight

– Viewshed

– Combined Viewshed

– Cumulative Viewshed

Viewing point

Calculating an inter-visibility matrixCalculating an inter-visibility matrix

Offset bOffset a

vv v nvnvnv

visible not visible

without offset b

with offset b