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How Can Data Be Georeferenced?
How Can Data Be Georeferenced?
When data (such as an old scanned map or other unreferenced data) is georeferenced, each location in that data is aligned with real-world coordinates.
So to start the whole process, you’re going to need some type of source with
real-world coordinates to align your unreferenced data against. The choice
of a source to use for the matching is important since you have to be able to
find the same locations on both the unreferenced image and the source. This
is crucial to georeferencing, since the process needs some common locations
to “tie” the unreferenced data to the source data. In the georeferencing procedure, these spots (where you can find the coordinates on the source data)
are referred to as control points (sometimes called Ground Control Points or
GCPs). Field work, such as surveying or GPS measurements (see Chapter 4) can
provide sources for these control points, but without going into the field, you
can also find common locations on the unreferenced image and the source to
use in the process.
Thus, the source should be something you can find the same locations on
as the unreferenced data. For instance, a low-resolution satellite image isn’t
going to be a good match with the detailed map of your campus buildings.
Trying to match individual locations like buildings to an image like this isn’t
going to work simply because the resolution is too low. A high-resolution image (where you can determine features of buildings) would be a more proper
choice for this specific task. Datasets such as road networks (see Chapter 8)
or orthophotos (see Chapter 9) are also good sources to use in finding these
common points. Also, your source data should be a similar projection to your
unreferenced data. If the projections are very different, then some features
will be forced to match up well and others will end up distorted. Whatever the
source, the georeferencing process will align an unreferenced image against
the projection and coordinates of the source map (Figure 3.3 on page 56).
Where to select the control points is the key to the whole georeferencing procedure. Since control points are the objects that are going to connect
to interact with Google Earth just like in the last
couple of labs, but you’ll also see special symbols
on the map representing the location of the georeferenced maps. Click on these symbols to see the
maps appear—properly georeferenced—in relation
to Google Earth’s imagery. Check out some of the
map icons for a variety of places around the globe
and see how this historic information synchs up
with modern boundaries and images.
The Rumsey Historical Map collection is also
available as a layer in Google Earth itself. To use the
maps as another Google Earth layer (like the roads
or buildings), open the Gallery options and then
open the Rumsey Historical Maps option, and click
on the Map Finder icons. You’ll see a new icon representing the maps appear in Google Earth that you
can use to view the georeferenced maps. There’s
also an “About the Maps” pop-up where you can
read more information about the map collection and
how these maps were scanned and georeferenced
for use in Google Earth.
control points point
locations where
the coordinates are
known—these are
used in aligning the
unreferenced image to
the source.
How Can Data Be Georeferenced?
How Can Data Be Georeferenced?
When data (such as an old scanned map or other unreferenced data) is georeferenced, each location in that data is aligned with real-world coordinates.
So to start the whole process, you’re going to need some type of source with
real-world coordinates to align your unreferenced data against. The choice
of a source to use for the matching is important since you have to be able to
find the same locations on both the unreferenced image and the source. This
is crucial to georeferencing, since the process needs some common locations
to “tie” the unreferenced data to the source data. In the georeferencing procedure, these spots (where you can find the coordinates on the source data)
are referred to as control points (sometimes called Ground Control Points or
GCPs). Field work, such as surveying or GPS measurements (see Chapter 4) can
provide sources for these control points, but without going into the field, you
can also find common locations on the unreferenced image and the source to
use in the process.
Thus, the source should be something you can find the same locations on
as the unreferenced data. For instance, a low-resolution satellite image isn’t
going to be a good match with the detailed map of your campus buildings.
Trying to match individual locations like buildings to an image like this isn’t
going to work simply because the resolution is too low. A high-resolution image (where you can determine features of buildings) would be a more proper
choice for this specific task. Datasets such as road networks (see Chapter 8)
or orthophotos (see Chapter 9) are also good sources to use in finding these
common points. Also, your source data should be a similar projection to your
unreferenced data. If the projections are very different, then some features
will be forced to match up well and others will end up distorted. Whatever the
source, the georeferencing process will align an unreferenced image against
the projection and coordinates of the source map (Figure 3.3 on page 56).
Where to select the control points is the key to the whole georeferencing procedure. Since control points are the objects that are going to connect
to interact with Google Earth just like in the last
couple of labs, but you’ll also see special symbols
on the map representing the location of the georeferenced maps. Click on these symbols to see the
maps appear—properly georeferenced—in relation
to Google Earth’s imagery. Check out some of the
map icons for a variety of places around the globe
and see how this historic information synchs up
with modern boundaries and images.
The Rumsey Historical Map collection is also
available as a layer in Google Earth itself. To use the
maps as another Google Earth layer (like the roads
or buildings), open the Gallery options and then
open the Rumsey Historical Maps option, and click
on the Map Finder icons. You’ll see a new icon representing the maps appear in Google Earth that you
can use to view the georeferenced maps. There’s
also an “About the Maps” pop-up where you can
read more information about the map collection and
how these maps were scanned and georeferenced
for use in Google Earth.
control points point
locations where
the coordinates are
known—these are
used in aligning the
unreferenced image to
the source.