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Chapter 3 Getting Your Data to Match the Map
There’s a research project in the Czech Republic
dedicated to (in part) georeferencing old historic
maps. To check out their work, open your Web
browser and go to http://help.oldmapsonline.org
to read about their current efforts. On the Website is
a link to their Georeferencing Tool, an online utility
for georeferencing historic data (also available at
http://www.georeferencer.org). Click on the link to
open the tool—you’ll see a scanned historic map on
the left-hand side and a source map on the right.
Zoom in on both maps to try and find some common
points, and double-click to create a control point.
Hands-on Application 3.2
Old Maps Online
will attempt to match up the next control point location with a spot you select
on the source. As you properly add some more control points, the fit between
the unreferenced data and the source should (hopefully) improve. Proper
selection of control points is critical for georeferencing since the procedure
needs to have some sort of real-world coordinates to match to, and it’s these
places that will be used to calculate coordinates for the other locations in the
image. Often, you’ll find that deleting a control point, adding another one,
and tinkering with the placement of others is necessary to get the best fit of
the data (see Hands-on Application 3.2: Old Maps Online for an online example
of placing control points in relation to historic maps). Once your data is properly aligned using control points, you can then transform it to have the same
coordinate system information as the source.
How Is Data Transformed to a
Georeferenced Format?
Take another look at Figure 3.3, which shows both the unreferenced campus
map and the source image. In order to make the unreferenced map properly
align with the source image, there are a number of actions that have to be taken with the map. First, check out the football stadium (Stambaugh Stadium)
in the upper left part of the map and the upper portion of the image. In order
to make it match from the map to the image, it would have to be turned about
45 degrees clockwise. Looking at the other features, the whole map would
have to be rotated some amount (since the image is oriented to the north
and the map is oriented to the northeast). Also, the map might have to be
stretched or skewed to match up with the image.
When unreferenced data is transformed or “ rectified”, it’s warped to
make it match up with the source. It can be rotated (turned, like changing the
orientation of the map), skewed (pulling the image, like at a slant), scaled
(altering the entire map to match the image), and/or translated (altering
rotated when the
unreferenced image
is turned during the
transformation.
skewed when
the unreferenced
image is distorted or
slanted during the
transformation.
scaled when the scale
of the unreferenced
image is altered during
the transformation.
translated when the
unreferenced image
is shifted during the
transformation.
Chapter 3 Getting Your Data to Match the Map
There’s a research project in the Czech Republic
dedicated to (in part) georeferencing old historic
maps. To check out their work, open your Web
browser and go to http://help.oldmapsonline.org
to read about their current efforts. On the Website is
a link to their Georeferencing Tool, an online utility
for georeferencing historic data (also available at
http://www.georeferencer.org). Click on the link to
open the tool—you’ll see a scanned historic map on
the left-hand side and a source map on the right.
Zoom in on both maps to try and find some common
points, and double-click to create a control point.
Hands-on Application 3.2
Old Maps Online
will attempt to match up the next control point location with a spot you select
on the source. As you properly add some more control points, the fit between
the unreferenced data and the source should (hopefully) improve. Proper
selection of control points is critical for georeferencing since the procedure
needs to have some sort of real-world coordinates to match to, and it’s these
places that will be used to calculate coordinates for the other locations in the
image. Often, you’ll find that deleting a control point, adding another one,
and tinkering with the placement of others is necessary to get the best fit of
the data (see Hands-on Application 3.2: Old Maps Online for an online example
of placing control points in relation to historic maps). Once your data is properly aligned using control points, you can then transform it to have the same
coordinate system information as the source.
How Is Data Transformed to a
Georeferenced Format?
Take another look at Figure 3.3, which shows both the unreferenced campus
map and the source image. In order to make the unreferenced map properly
align with the source image, there are a number of actions that have to be taken with the map. First, check out the football stadium (Stambaugh Stadium)
in the upper left part of the map and the upper portion of the image. In order
to make it match from the map to the image, it would have to be turned about
45 degrees clockwise. Looking at the other features, the whole map would
have to be rotated some amount (since the image is oriented to the north
and the map is oriented to the northeast). Also, the map might have to be
stretched or skewed to match up with the image.
When unreferenced data is transformed or “ rectified”, it’s warped to
make it match up with the source. It can be rotated (turned, like changing the
orientation of the map), skewed (pulling the image, like at a slant), scaled
(altering the entire map to match the image), and/or translated (altering
rotated when the
unreferenced image
is turned during the
transformation.
skewed when
the unreferenced
image is distorted or
slanted during the
transformation.
scaled when the scale
of the unreferenced
image is altered during
the transformation.
translated when the
unreferenced image
is shifted during the
transformation.