1. Vertical Control Introductions Purpose for presentation
Have you ever considered what it means to say that the Dworshak Dam , the highest straight-axis concrete dam in the Western Hemisphere , has a height of 1581 feet? Where is this measured from -- the center of the Earth, the surface of the Earth, or somewhere else (MSL)? (717’ from base) How do we define zero height or zero elevation?

2. Vertical Datums
Heights are a vertical measurement from some point or surface to the top of the surface being measured. The height of a desk, a person, or a shelf on a wall are all commonly measured as a distance from the floor. In this sense, the floor is a vertical datum or a plane of reference from which the heights of various things can be measured and because they all used the same datum, their heights can be compared and used in logical way relative to one another.
To measure the height of things outdoors, surveyors establish a datum for a plane of reference just as the floor is used in the example above. Because nothing outdoors is perfectly flat like a floor, the datum “plane” can be visualized as an imaginary plane located at some specified vertical position. In most common terms “sea level” has such a meaning to many people. The height of a mountain peak is expressed as some vertical measurement “above sea level” or sometimes simply the “elevation” of the peak. For most non-technical purposes, this expression and concept of elevation is completely adequate.
The vertical datum is a collection of specific points on the Earth with known heights either above or below mean sea level. Near coastal areas, mean sea level is determined with a tide gauge. In areas far away from the shore, mean sea level is determined by the shape of the geoid.

3. Not Influenced by Gravity
Vertical Datums
A vertical datum is used for measuring the elevations of points on the earth's surface.
Geodetic Datums (NGVD29, NAVD88)
Hydrologic Datums (LWRP, IGLD)
Tidal Datums (MSL or LMSL, MLLW, MHW)
Local (Cario, Memphis Datum)
Ellipsoid (Space-based) Surfaces
Influenced by Gravity
For engineering, science, and most technical purposes, much more specificity and precision must be employed to adequately describe the height of things. For example, the surface of the earth is curved and not flat, so the analogy of a floor as a datum only works in localized situations. Knowing the relative elevation of two things separated by a large distance requires accounting for the curvature of the earth. Additionally, the concept of using sea level as a basis for vertical datums is not so simple. Tides must be considered as well as the fact as sea level is not the same everywhere. Additionally, sea level at any given point changes over time. The curvature of the earth is not uniform and mathematical approximations used to describe the curvature are imperfect.
Not Influenced by Gravity

4. Orthometric Heights Comparison of Vertical Datum Elements
Vertical Datums
Orthometric Heights Comparison of Vertical Datum Elements
NGVD NAVD 88
DATUM DEFINITION TIDE GAUGES FATHER’S POINT/RIMOUSKI
IN THE U.S. & CANADA QUEBEC, CANADA
BENCH MARKS , ,000
LEVELING (Km) , ,001,500
TIDAL EPOCH None
GEOID FITTING Distorted to Fit MSL Gauges Best Continental Model
This shows the differences in the components of the vertical datums.
The datum is not just the values, but the way the computations are done.
NGVD29 is superseded by NAVD88
After NAVD88 was established, most of the technical work ongoing in the country continued to reference elevation to NGVD29. There were many reasons for this including familiarity with the existing datum, new work referenced to the old datum was also easily reference to old work that was already constructed in NGVD29. Additionally, converting all elevation values presently in NGVD29 to NAVD88 was viewed to be disruptive, expensive, and provide little value. However, as time marched on, reasons for referencing new work to NAVD88 became more convincing and over time, more and more work was referenced in the new datum.

5. National Vertical Network
Early 1900s
SLD29
NGVD29
NAVD88
The North American Vertical Datum of 1988 is referenced to 1 tide gage on the St. Lawrence Seaway
1,000,000+ km
The Sea Level Datum of 1929 was referenced to 26 tide gages in the US and Canada
~100,000 km
The Sea Level Datum of 1929’s name was changed to the National Geodetic Vertical Datum in 1973
It turns out that MSL is a close approximation to another surface, defined by gravity, called the geoid, which is the true zero surface for measuring elevations. Because we cannot directly see the geoid surface, we cannot actually measure the heights above or below the geoid surface. We must infer where this surface is by making gravity measurements and by modeling it mathematically. For practical purposes, we assume that at the coastline the geoid and the MSL surfaces are essentially the same. Nevertheless, as we move inland we measure heights relative to the zero height at the coast, which in effect means relative to MSL.
Leveling network in the early 1900s
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Table showing the progression of network density and adjustments
Definition: NGVD29 – The National Geodetic Vertical Datum of 1929… (21,000+ k leveling)
The Sea Level Datum of 1929 was the vertical control datum established for vertical control surveying in the United States of America by the General Adjustment of The datum was used to measure elevation or altitude above, and depression or depth below, mean sea level (MSL).
Mean sea level was measured at 26 tide gauges: 21 in the United States and 5 in Canada. The datum was defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. The adjustment required a total of 66,315 miles (106,724 km) of leveling with 246 closed circuits and 25 circuits at sea level.
Since the Sea Level Datum of 1929 was a hybrid model, it was not a pure model of mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed the National Geodetic Vertical Datum of 1929 (NGVD 29) in 1973.
The NGVD 29 was subsequently replaced by the North American Vertical Datum of 1988 (NAVD 88) based upon the General Adjustment of the North American Datum of 1988.
NAVD88 is tied or held fixed at one bench mark (Rimouski at Father’s Point along the St. Lawrence Seaway.

6. Contours showing the difference between NAVD88 and NGVD29
Contours showing the difference between NAVD88 and NGVD29. This chart does not show the difference between elevations.

7. Referenced to 1 Tide Gage Referenced to 26 Tide Gages
NAVD 88
Referenced to 1 Tide Gage
(Father’s Point)
NGVD 29
Referenced to 26 Tide Gages
-23 cm
-11 cm
4 cm
125 cm
70 cm
85 cm
102 cm
NAVD88 minus LMSL( )
NGVD29 consisted of the national vertical network referenced to 26 tide stations along the coast of North America
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That network was warped or twisted to fit those tide stations. The difference between the geoid (what we run levels on) and LMSL is not the same for every tide station due to prevailing winds, ocean current, etc. It is that difference that warped the network.
NAVD88 utilized additional leveling data along with gravity measurements (to measure the geoid) and was referenced to only one tide station on the St. Lawrence Seaway. As you can see in the graphic, the network was unwarped which changes those reference mark’s elevations (differences between NAVD88 and the local mean sea level shown) This is not the difference between NAVD88 and NGVD29 but it is a representation of what happened.
The relationships between LMS and NAVD88 as measured by IPET

8. Vertical Segment of the NSRS
NOAA's National Geodetic Survey (NGS) defines and manages a national coordinate system.
This network, the National Spatial Reference System (NSRS), provides the foundation for our spatial infrastructure
NGS develops Federal standards for geodetic surveys and helps to coordinate surveying methods.
NGS State Geodetic Advisors are stationed in several states to work with local communities to expand surveying capabilities.
This illustrates the extent of the vertical component of the NSRS at the time of the NAVD88 adjustment.
The National Spatial Reference System (NSRS) exists as a collection of discreet geodetic elements. Horizontal positions (latitude and longitude, State Plane Coordinates) referenced to the North American Datum of 1983 (NAD 83), and elevations referenced to the North American Vertical Datum of 1988 (NAVD 88) define the major components of the NGRS. Both datums are the latest definitions of systems that began with the first geodetic surveys conducted by the Survey of the Coast, founded in 1807 (later renamed the Coast Survey-1836, U. S. Coast and Geodetic Survey-1878, and National Ocean Service-1970). NGS is responsible for most of NOS' geodetic mission. These geodetic systems were developed independently with little regard to being correlated. That is, a survey monument that provided accurate horizontal coordinates was not normally connected to the vertical network, nor were vertical points related to the horizontal reference network. Today (1994), the NGRS contains information on over 300,000 horizontal control points and 600,000 vertical control points.
Click on advisor's name to bring up their presentation
It must be named ngs.ppt .

9. Changing Elevations in a Dynamic Environment
PBM ALCO
Changing Elevations in a Dynamic Environment

10. Changing Elevations in a Dynamic Environment

11. Additional Information Available at:
Questions?
Additional Information Available at: