raster data sources: paper maps & aerial photographs

CS 128/ES 228 - Lecture 10a 1

Raster Data Sources:Paper maps &

Aerial photographs

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Georgian’s First Law of GISTry to use

somebody else’s data before you

even think of generating your

own.

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Data sources: overviewRaster sources: Paper maps Aerial photographs Satellite images

Vector sources: Digitized maps Surveying Global positioning system

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Paper maps

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Using paper maps in a GIS1. Scan to image file (usually JPEG or GIF)

2. Georeference the image to the GIS coordinate system

3. (If desired) digitize the features in the image to generate 1+ vector layers

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Georeferencing raster images Spatial coordinates may be absent or purely

map coordinates (i.e. inches from one corner)

Control points: point features visible on both the image and the map

Linear or nonlinear transformations

“Rubber sheeting”

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Affine transformations Translation

Rotation

Scaling

Skew

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Georeferencing in ArcMap - 1

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Georeferencing in ArcMap - 2

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Georeferencing in ArcMap - 3

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Georeferencing in ArcMap - 4

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Georeferencing in ArcMap - 5

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Georeferencing in ArcMap – 5b

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Georeferencing in ArcMap - 6

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Georeferencing in ArcMap - 7

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Georeferencing in ArcMap - 8

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Georeferencing in ArcMap - 9

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Digitizing raster layersDigitizing table high resolution (0.001”) either point or stream

mode paper shrinkage/

expansion data in “table

coordinates” – need to convert to map coordinates

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“Heads up” digitizingTracing on computer

monitor: many scanned (raster)

file formats supported poorer resolution, but

uses less specialized equipment

best for adding small # features or updating a file

uses coordinate system of image or base map

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Aerial photographs HUGE amount of

detail VAST number of

photographs are available, often for free

Digitizing and photo-interpretation can produce vector layers and attribute data

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Photogrammetry Originally, the science

(or art?) of interpreting aerial photographs

Stress on quantitative measurements

Now includes analysis of digital images from many sources

Image from Avery. Interpretation of Aerial Photographs.

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ScaleDetermine from: Plane altitude

RF = lens focal length altitude of plane

Known ground features

Top image from Avery. Interpretation of Aerial Photographs.Bottom images from Ben Meadows catalog (L), Olean NW DOQQ ®

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Perspective Vertical:

- orthogonal perspective- planimetric map data

Oblique: - high oblique

(includes horizon) - low oblique (no horizon)

Image from Avery. Interpretation of Aerial Photographs.

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Planimetric view Perfectly vertical

(orthogonal) perspective

All features in correct horizontal positions

Impossible unless at infinite height

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The principle point Point directly under

camera lens (‘nadir’)

Elevated objects lean away from PP

Depressed objects lean toward PP

Causes horizontal image displacement

Images from Avery. Interpretation of Aerial Photographs.

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Vertical relief -> displacement Transmission line is

straight - why does the line appear straight in one photo and jagged in the second?

In left photo, line is ~ on nadir; in right photo, the line is far from nadir

Image from Avery. Interpretation of Aerial Photographs.

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Image displacement: Source of error

in horizontal locations, but

Permits estimation of feature elevations

stereoscopic

parallaxImage from Avery. Interpretation of Aerial Photographs.

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Stereoscopic photo pairs

Image from Avery. Interpretation of Aerial Photographs.

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Stereoscopes need pair of

overlapping photos

different principle points results in parallax

used to create topographic contours

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Rectification of aerial photographsRectification: process of geometric correction

that turns an aerial photograph into a planimetric (map-like) image

Problems: Earth curvature lens distortion camera tilt terrain relief

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Rectification process1. Scan aerial photograph at high resolution

2. Locate ground control points on scanned image: ≥3 for affine transformation ≥5 for rubbersheeting

3. Combine with digital elevation model (DEM) to correct relief displacement

4. Rectify to a ground coordinate system

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Relief distortionObjects at different distances form

the lens will be distorted

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Urban areas: building tiltIn urban areas, tall buildings

seem to lean toward the principal point of the photograph

Corrected by building a digital terrain model (DTM) of each building

Permits virtual reality “flyovers”

Thorpe, A. Digital orthophotography in New York City. www.sanborn.com/Pdfs/Article_DOI_Thorpe.pdf

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Result: digital orthophotograph USGS: DOQQs

NYS GIS Clearinghouse

Or, new aerial photos & image rectification ($$$)

Wind Cave N P Vegetation Survey? Sure (tax $$)No!!!CS 128/ES 228 Course Project?