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## Frequently Asked Questions on Geodetic Vertical and Water Level Datums for Engineers

1. What is JavaScript?
JavaScript is a scripting language originally developed by Netscape to add interactivity and power to web documents. It is purely client side, and runs completely on the client's browser and computer.

2. What is the difference between plane and geodetic datum reference systems?
A plane reference system uses a flat surface as the origin or 0 elevation. A flat surface does not account for the curvature of the earth’s surface and should not be used for large projects. Typically these small surveys may not even be referenced to the NSRS and might use a local mark with an assumed elevation (i.e. 100.0’) which would be considered a local datum.

Geodetic datums define the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth. Hundreds of different datums have been used to frame position descriptions since the first estimates of the earth's size were made by Aristotle. Datums have evolved from those describing a spherical earth to ellipsoidal models derived from years of satellite measurements.

3. What is the geoid? What is the ellipsoid? Why do we need both reference systems.
The geoid is an equipotential gravitational surface that is approximated by mean sea level and is everywhere perpendicular to the local direction of gravity. The geoid is affected by the distribution of the earth's mass and rotation, hence it is an irregular surface. The ellipsoid is a mathematically defined surface obtained by revolving an ellipse around the earth's polar axis. The dimensions of the semi-major and semi-minor axes of the ellipse are chosen to give a good fit of the ellipsoid to the geoid over a large area, such as North America or Europe. Approximating the earth's shape as an ellipsoid occurred as early as the nineteenth century. In current practice an important reason for both reference systems is that positions determined using the GPS are relative to the ellipsoid and elevations determined by conventional leveling are relative to the geoid. A geoid model is used to determine the difference between the two surfaces.

4. What are orthometric heights?
An orthometric height is the distance above or below the geoid to the earth's surface, structure, or benchmark and is commonly called “elevation”. An ellipsoid height is the distance above or below the mathematical model of the earth (that best fit squashed sphere or GRS80). Note that the vector describing the orthometric height is parallel to the local gravity vector whereas the vector describing the ellipsoid height is normal to the ellipsoid surface.

5. What are tidal datums?
A tidal datum is a surface defined by a particular phase of the tide. Examples of tidal datums are Mean Higher High Water (MHHW), Mean Tide Level (MTL), Mean Sea Level, and Mean Lower Low Water (MLLW). Tidal datums are computed using observations acquired at a specific tide station and are valid only at that particular station. A datum located in a water body is considered tidal if the variation in water level is sufficiently predictable based upon the phases of the moon and sun. Otherwise, it is considered a non-tidal datum.

6. What is a tidal epoch?
Tidal variations can be considered as comprised of periodic and apparent secular trends. A specific 19-year period based upon the Metonic cycle is selected so that all tidal datum determinations will have a common reference period. This period is termed a tidal epoch. For tide stations located in the United States and its possessions, this period is called the National Tidal Datum Epoch (NTDE). The present NTDE is the period 1983 through 2001.

7. Why is it important to use a particular tidal epoch?
Tidal records typically contain secular trends in addition to the periodic variations. Over time, the secular trend can alter the relative elevation between tidal and related terrestrial datums. When comparing elevations which are based upon tidal datums, one should insure that common epochs are used in order to account for the effects of secular trends. Moreover, the presence of a secular trend can cause changes in related tidal datums, such as MLLW and MHHW.

8. How great a distance from a tide station can I move and still have a valid tidal/terrestrial datum relation?
In the strict sense, a tidal datum is applicable only at the location of the tide station. This is because tides and therefore tidal datums are affected by the gradient of the geoid. In order to validly extend a tidal datum over some horizontal distance, the geoid gradient must be negligible compared to the projected horizontal distance. This requires knowledge of the local geoid gradient, which can be determined by gravity measurements or establishing a second tide station. For example, if a particular tidal datum is known to be accurate to within one centimeter, and it is desired to maintain that degree of accuracy, then the greatest horizontal distance a tidal datum can be projected would be the geoid gradient times the distance to equal one centimeter. A cautionary note - some coastal locations may have significantly different tidal characteristics within a short reach of coastline, such as a transition from a semi-diurnal regime to a mixed tide regime. It is inadvisable to attempt to extend tidal/terrestrial relations across such transitions even though the distance is small.

9. Why not just use National Geodetic Vertical Datum of 1929 (NGVD29) for everything?
During the early part of the twentieth century it was assumed that global sea level was everywhere the same. In 1929 a general adjustment of benchmarks was conducted to obtain a best fit of mean sea level observations at 26 tide stations in the United States and Canada. It was subsequently discovered that global mean sea level is not the same throughout the oceans. Moreover, the national benchmark network has been extensively expanded subsequent to 1929 and both subsidence and isostatic rebound have occurred in the continental US. The result is, that based upon the North American adjustment of 1988 (NAVD88), the Atlantic coastline is about 30cm below NGVD29 and the Rocky Mountains and westward to the Pacific coast is about 100cm higher than NGVD29. Use of NGVD29 would result in gross elevation errors everywhere except perhaps locations in the central Mississippi Valley.

10. How does one determine the relationship between NGVD-1929 and NAVD-1988?
The methodology used to shift historical survey data to NAVD88 (2004.65) will vary depending upon many factors such as time, funds, accuracy requirements, etc. Generally there are four methods to determine the datum/epoch shift.

a. Field measurements with known historical elevation: This method will yield the most accurate values based on the historical reference marks. The reference marks will need to be recovered and occupied/surveyed using the guidelines in NGS Publication 58. The difference between the elevation used for the original survey and the elevation established from the new network will directly tie in the old work to the latest control. This will not account for any differential subsidence that occurred between the reference mark and the survey positions.

b. Field Measurements without known historical elevation: When the reference benchmark is not recorded and unknown, some assumptions will be required such as what mark was used and what its elevation was. Again follow the procedures in NGS 58 to establish new elevations on the reference mark. The historical elevation will have to be assumed based on what was available at the time of design. The difference between the assumed historical elevation and the newly established elevation will be used to shift the survey to the new datum/epoch.

c. Common published marks in survey area: When time and money are constraints, the closest marks with published elevations in both datum/epochs can be used to determine an average shift for the area. This method contains many assumptions and therefore is the least accurate but may be of some use for projects that don't require accuracy.

d. CORPSCON: This method does not account for subsidence or the change in elevation from epoch to epoch. CORPSCON model was also tied to the published elevations at the time the conversion model was created which contained errors associated with the already deteriorating elevation accuracies. This method should not be used for anything other than a simple datum shift, keeping in mind that subsidence is not accounted for.

The most accurate method to accomplish that is to use GPS to re-observe each and every benchmark used for an old survey of interest. There is absolutely no way to compute it; there are no computer programs that are reliable for such a conversion; old benchmarks must be re-occupied to perform a re-determination of the current elevation of the mark. Many parts of the United States are areas of relatively stable elevations. The entire State of Louisiana is an area of crustal motion – we subside different amounts in different places and at different times! In fact subsidence has been detected as far north as St. Louis. The speed we subside changes at the same spot, and the speed of subsidence differs from spot to spot. We are unable to predict crustal motion exactly, whether it’s in Louisiana or in Tokyo or in Southern California.

11. How do GPS determined elevations fit into this picture?
GPS positions (elevations) are relative to the ellipsoid and are termed ellipsoid heights whereas elevations determined using conventional leveling methods are referenced the geoid. To transform GPS determined elevations to other coordinate systems, at least one point in the survey must have known coordinates and datum in the desired system. Although hand-held CA code GPS receivers report elevations, these elevations are not sufficiently accurate for survey purposes. Carrier-phase based differential GPS elevations may be sufficiently accurate provided at least one benchmark of known elevation is included in the survey.

12. What is CORS?
The Continuously Operating Reference Stations (CORS) is a network of GPS receivers that collect and transmit GPS measurements to the National Geodetic Survey where it can be retrieved via the Internet and used for activities ranging from precise geodetic surveys to real-time vehicle navigation. The CORS network is the backbone of the On-Line Positioning User Service (OPUS) and provides an effective means to relate our projects to the National Spatial Reference System.

The vast majority of the CORS are owned and operated by a collection of more than 80 organizations representing various federal, state, and local government agencies, as well as various academic and commercial institutions.

Additional information is available at: http://www.ngs.noaa.gov/INFO/OnePagers/NationalCORS.pdf