![]() ![]() The original datum was called the “Sea Level Datum of 1929” and it was the first vertical (elevation) datum for an entire continent in the history of the world. Once thought to be close to Mean Sea Level (MSL), it was within a couple of feet for most of the northern rim of the Gulf of Mexico. We are unable to predict crustal motion exactly, whether it’s in Louisiana or in Tokyo or in Southern California. The speed we subside changes at the same spot, and the speed of subsidence differs from spot to spot. 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. Many parts of the United States are areas of relatively stable elevations. 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. 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. This method should not be used for anything other that a pure datum shift keeping in mind that subsidence is not accounted for. 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 contains many assumptions and therefore is the least accurate but may be of some use for projects that don't require accuracy.ĬORPSCON: This method does not account for subsidence or the change in elevation from epoch to epoch. ![]() The difference between the assumed historical elevation and the newly established elevation will be used to shift the survey to the new datum/epoch.Ĭommon 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. The historical elevation will have to be assumed based on what was available at the time of design. Again follow the procedures in NGS 58 to establish new elevations on the reference mark. This will not account for any differential subsidence that occurred between the reference mark and the survey positions.įield Measurements w/o 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. 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. The reference marks will need to be recovered and occupied/surveyed using the guidelines in NGS Publication 58. Generally there are four methods to determine the datum/epoch shift.įield Measurements w/ Known Historical Elevation: This method will yield the most accurate values based on the historical reference marks. The methodology used to shift historical survey data to NAVD88 (2004.65) will vary dependent upon many factors such as time, funds, accuracy requirements, etc. Benchmarks subside at very rapid rates in the coastal areas of Southeast Louisiana and at much slower rates near Alexandria and Deridder, LA. How long an epoch is used depends on where the benchmarks are located within the New Orleans District. Although the benchmarks continuously change with time because of the inexorable subsidence, benchmarks of a certain epoch are used by surveyors and engineers for a short period of time before their elevation values have changed beyond standard engineering specifications. Since the Datum does not change and elevations do change with subsidence, then the values of benchmark elevations do in fact change with time.Īn Epoch is a collection of elevation benchmarks that have been determined for a given instant in time. However, elevations change in Louisiana with time because of subsidence. A modern Datum does not change with time, and it stays the same. Although it is not the same as local mean sea level, it is usually within a foot or so of that as it varies from place-to-place because of tidal variations, winds, currents, and river stages in South Louisiana. Commonly associated with the concept of local mean sea level, the current system of elevations in North American is called the North American Vertical Datum of 1988. In the context of elevations, a Datum is a reference system that is used to compare elevations of various places to a certain height. ![]()
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