Before you can match your scanned image to
geospatial data you must put it into a coordinate system. This process matches
features in your photograph to real world coordinates on the ground. Some
examples of reference systems are Latitude and Longitude, Universal Transverse
Mercator (UTM), and State Plane Coordinate System. If you have specific
geospatial data you want to use then it would be best to reference your image to
the coordinate system of your data. For a discussion of coordinate systems
please see:
1.
Geocoding Spatial Data, by Keith Clarke
2.
GIS terms
Glossaryfrom GISdevelopment.net
3. The Idrisi
manual has a very good discussion of coordinate
systems and datums for
georeferencing.
Finding Ground Control Points (GCPs)
To georeference an image you need GCPs which are
visible in the photographs. Some examples of good GCPs are road intersections,
stone wall boundaries, building corners, and solitary trees. These points will
be used to “tell” the GIS software:
where your image is in the world
how to correctly orient the photograph
correct for errors in photo-geometry.
These errors are probably caused by the inherent
problems of taking aerial photographs, such as airplane tilt and problems with
the lens. The better your GCPs the better your resulting image will be
referenced to the real world.
The number of
GCPs you choose will depend on the amount of distortion in your photograph and
your desired level of accuracy. The process of registering your photograph
applies a mathematical formula to each pixel in the photo. The process of
rectification can be thought of making a regression equation that says where a
image coordinate corresponds to real world coordinates. The simplest formula is
a linear equation, which does not distort the picture but can not correct any
photo-geometry distortion except for skew. Higher order (more complex) equations
can correct more serious cases of photo-geometry distortion but they can also
seriously distort your final image. As you make the equation more complex
you have to add more GCPs.
A linear equation requires a minimum of 3
points, a second-order equation requires 6 points, and a third-order equation
requires a minimum of 10 points. In general you should find at least double the
number points so that you can discard bad points and you can also lower the
error in fitting the equation.
At the map library we have a collection of maps
called USGS Topographic Quadrangle Sheets
(quad sheet). These maps cover the entire U.S. at a scale of 1:24,000. By using
a coordinate grid and a quad sheet, you can get coordinates for GCPs in
Lat/Long, UTM, and State Plane. The map library has a reference map for New
England which gives the name for the quad sheets. Once you have your quad
sheet(s) you can put the coordinates into a GIS and georeference your image. To
learn how to get coordinates off a quad sheet please read: CHOOSING GCPS and
READING COORDS OFF A COORDINATE SHEET.
Getting the coordinates
into a GIS
The rest of the instructions for georeferencing an
aerial photograph assume that you know the basics of
Idrisi. We have put together an
Idrisi page on MAGIC and there is a complete set of manuals in the map
library.
The first step in georeferencing your image is
to open a Correspondence File. This file will be used by Idrisi to reference
your image. The best idea is to call the file <YourImageName>.cor, where
YourImageName corresponds to the name of your scanned image. Open the Windows
Notepad (or any ASCII text editor). The format of the file is as follows
The first line of the file denotes how many GCPs
there are in the file (5 in this instance). The following lines are:
originalX originalY newX newY
Where the original X and Ys correspond to the
column and row numbers of the GCP from the photograph after it has been
imported into Idrisi. The new X and Ys correspond to the coordinates for the
same GCP read off the quad sheet (CT State Plane in this instance).
Save the .cor file and begin the Idrisi
RESAMPLE module. This process will put your photograph into real world
coordinates.
