Do you ever think about how location information impacts your work. Accurate
positions and elevations are critical to construction projects,
flood plain mapping, scientific endeavors, and much much more.
Surveyors use scientific instruments and field procedures
to collect information about positions, distances and elevations.
Then this information is used to create geodetic datums,
reference frames used at a national level to assure consistency across
mapping applications.
Geodetic reference frames have a long history in the United States,
dating back to the 1800s. In 1807,
President Jefferson signed the act that established the Survey of the Coast.
Work started by measuring angles and distances between points across the country.
This formed a triangulation network that continue to be densified over the years.
This network would become the basis of the future horizontal datums.
To create the vertical datum, a highly accurate surveying technique
called geodetic leveling was used to measure height differences across the country.
During a twenty year period beginning in 1877,
a "level line" was surveyed across the entire United States.
As the network of level lines across the country expanded,
this became the basis for future vertical datums.
There were many tools and instruments used to complete these geodetic surveys,
and some are still used today. For example thoedolites have been used for
centuries to measure angles.
A variety of other tools have been used over the years to measure distances
including chains bars and invar tapes...
continuing to today's Total stations which can measure angles
as well as electronically measure distances.
The tools for accurate elevation measurement have also remained fairly
consistent over the years
with the use of level rods and a sighting instrument to measure the
height differences between two points.
Today a laser may replace the use a telescope
but the approach remains the same.
Over many decades, surveyors used these tools to create a tremendously
accurate reference system across the United States
with many applications and benefits.
The North American datum of 1983, called
NAD 83, is the horizontal datum for the United States
and much of NAD 83 is based on the extensive triangulation network
established over decades.
The North American vertical datum of 1988,
or NAVD 88 is based on an adjustment of leveling observations from across the country.