PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
February 2014
117
T
he U. S. Global Positioning System (GPS)
satellites have revolutionized surveying and
mapping activities. First-order astronomical
theodolites are now used as bookends or displays
in museums. Little classical triangulation is done
outside of academic instruction because of the
enormous cost-savings afforded by differential
GPS techniques. However, the fruits of centuries
of detailed
and largely reliable
classical surveys
are the basic foundation of today’s national
topographic maps existing in every country
throughout the world. When we venture into a
“new” mapping project, there is some pre-existing
survey data that will have to be incorporated
into that data set. Although a GPS-controlled
project is largely free of systematic error when
properly executed, the prospect of quantifying
the systematic error of an older data set and
incorporating that older data into the new
GRIDS
&
DATUMS of
system can be daunting. A successful mapping project
depends on the merger of the old with the new. An under-
standing and knowledge of past practices, techniques, and
reference systems is the pre-requisite to that success.
The primary coordinate reference system of interest is the
DATUM. The classical geodetic horizontal datum starts at
a particular point. The North American continent has one
major classical horizontal datum, which is the North Amer-
ican Datum of 1927. The starting point is defined at Meades
Ranch, Kansas. Most datums have their historical origins at
an astronomical observatory, mainly because when geodetic
reference systems were originated, the best-known position
in a region was at that observatory. That observatory also
had a “mire” or reference point on the horizon with a known
azimuth from true north (North Celestial Pole). With a
known direction reference and a known position, physically
measuring a distance to another point on the ground allowed
the computation of another known position (latitude and lon-
gitude) with reference to the starting point or datum point.
That’s how all datums started. The observatory for NAD 1927
was not in Kansas but on the East Coast, and was used for
