cs 128/es 228 - lecture 10a1 raster data sources: paper maps & aerial photographs

CS 128/ES 228 - Lecture 10a 1

Raster Data Sources:Paper maps &

Aerial photographs


Georgian’s First Law of GIS

Try to use somebody else’s

data before you even think of

generating your own.


Data sources: overview

Raster sources: Paper maps Aerial photographs Satellite images

Vector sources: Digitized maps Surveying Global positioning system


Paper maps


Using paper maps in a GIS

1. 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


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”


Affine transformations

Translation

Rotation

Scaling

Skew


Georeferencing in ArcMap - 1


Georeferencing in ArcMap – 5b


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


“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


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


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.


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 ®


Perspective Vertical:

- orthogonal perspective- planimetric map data

Oblique: - high oblique

(includes horizon) - low oblique (no horizon)



Planimetric view

Perfectly vertical (orthogonal) perspective

All features in correct horizontal positions

Impossible unless at infinite height


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.


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 displacement:

Source of error in horizontal locations, but

Permits estimation of feature elevations

stereoscopic parallax



Stereoscopic photo pairs



Stereoscopes

need pair of overlapping photos

different principle points results in parallax

used to create topographic contours


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


Rectification process

1. 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


Relief distortion

Objects at different distances form the lens will be distorted


Urban areas: building tilt

In 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

http://www.th.gov.bc.ca/seatosky/RFQ/flyovers.htm

http://www.th.gov.bc.ca/seatosky/RFQ/flyovers.htm


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?

cs 128/es 228 - lecture 10a1 raster data sources: paper maps & aerial photographs

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