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.