16 SERVO 05.2017
about the latitude and longitude of a point on the surface
of the globe, not all latitudes and longitudes are equal.
Where on the face of the planet a specific lat/long pair
corresponds to is dependent on the datum — or reference
system — used to locate it. Taking coordinates from
someone and locating them with a different datum can
result in errors of hundreds of meters!
Why are there different datums then? The Earth is a
very complex and bumpy shape; a datum that does an okay
job of describing the whole Earth may do very poorly in
certain locations. A datum made to work very well in a
given region will likely do poorly covering the entire globe.
The common datums encountered in North America
are NAD27/83 and WGS84. The North American
The datum is still used by many government agencies,
but the more advanced WGS84 datum is increasingly
common. The World Geodetic System (WGS) datum was
introduced in 1960, but the 1984 datum (with small
updates in 2004) is now used in many applications. The
WGS datum is based on the location of the center of the
mass of the Earth and is believed to be accurate to less that
2 cm. (Stop and think about that for a second.)
Finally, for local work that only takes place in a small
region, the Universal Transverse Mercator (UTM) coordinate
system is used instead of latitude and longitude. The UTM
system is a simple Cartesian grid or x,y position of a feature
in a certain zone. The Earth has been divided into 60 zones,
each consisting of a six-degree band of longitude extending
from 80° S to 84° N. The contiguous US spans zones 10-20
N. If you are really interested in the origin of datums and
coordinate systems, there are plenty of resources online,
but for now just know that it doesn’t really matter which
datum or coordinate system you use as long as it is
specified. For photographs, we can stick with the much
simpler coordinate system of Cartesian coordinates.
Generally, images are considered to have their origin at the
upper left corner of the image (Figure 9), but it’s always a
good idea to check depending on the specific piece of
software you are using.
It’s now time to create our GCP file. First, download
and install Google Earth ( https://www.google.com/
earth) and QGIS ( www.qgis.org). Open up Google Earth
and type in the location of your survey to get in the right
area of the globe (Figure 10). Next, manually pan and
zoom to find your specific area and start looking for good
control points. I found several options (Figure 11), and
Figure 11: Zooming into the mapping area a bit closer, I picked a
number of candidate ground control points (yellow stars). The
corners of buildings seemed to work very well, as did features like
the edges of road intersections.
Figure 9: Generally, we consider the origin of an image’s coordinate
system to be at the upper left corner of the image, with the x axis
values increasing to the right and y axis values increasing downward.
Figure 10: Searching the town name gets us into the right
region to search for the mapping area.