Surveying FAQs

• HOW DO I CONVERT NGVD29 TO NAVD88?

  The methodology used to shift historical survey data to NAVD88 (2004.65) or (2006.81) will vary dependent upon many factors such as time, funds, accuracy requirements, etc. Generally, there are four methods to determine the datum/epoch shift.

  Field Measurements w/ Known Historical Elevation: This method will yield the most accurate values based on the historical reference marks.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.

  Field 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. 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.

  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.

  CORPSCON/VERTCON: This method does not account for subsidence or the change in elevation from epoch to epoch. The VERTCON in 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 in Louisiana because subsidence is not accounted for, and there is no fix possible.

  The most accurate method to accomplish a vertical datum shift 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.

• HOW DO I CONVERT OLD SURVEYS TO NAVD88?

  The most accurate method to accomplish that is 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. Everywhere in the State of Louisiana is an area of crustal motion – we subside different amounts in different places and at different times! 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.

  To obtain a current elevation at an arbitrary spot or at a benchmark in Louisiana, the procedure requires the services of a Registered Land Surveyor that uses a geodetic-quality dual-frequency survey-grade GPS receiver with a ground plane antenna & fixed-height tripod. That type of contraption is very different from the GPS receivers one can purchase at a department store or fishing & sporting goods emporium.

  When the North American Datum of 1988 (NAVD88) was published for South Louisiana in 1992, some benchmarks that had NGVD29 values were included in the short list of benchmarks that had new NAVD88 elevations. For a few years, a computer program called VERTCON, sometimes used within another program called CORPSCON was used to convert those specific areas from one old datum to the new datum. That relation is no longer possible, since the surface of the Earth has changed so substantially since then due to subsidence. If that software is used for South Louisiana during the 21st century, the answers are guaranteed to be wrong.

• MY HANDHELD GPS GIVES ME BAD ELEVATIONS. WHY?

  All GPS receivers provide ellipsoid heights; they do NOT provide elevations.

  The difference between zero elevation on the North American Datum of 1988 (NAVD88) and zero ellipsoid height for South Louisiana is about twenty seven meters = 88.5 feet. Handheld GPS receivers (consumer grade), receive single frequency Coarse Acquisition code (CA code), where with Selective Availability turned off, has a nominal positional accuracy under good conditions of about ±15 feet in the horizontal.

  Since the vertical accuracy of Handheld GPS receivers is about seven times worse than horizontal positions, 15 × 7 = 105 feet + 88.5 feet = ±194 feet vertical accuracy under good conditions!

  That’s not bad for a $99 GPS receiver. For about $12,000 one can purchase a dual-frequency geodetic-quality GPS receiver that can provide ±2 centimeter accuracy over two days’ observations of at least 30 minutes each day, depending on baseline length. Of course, post-processing will require additional computer time to reduce the observed ellipsoid height to an elevation referenced to NAVD88. That is not possible with a handheld GPS receiver.

• WHAT KIND OF GPS RECEIVER DO I NEED?

  The Global Positioning System was designed by the U.S. Department of Defense to be a military system. Academic institutions throughout the world figured out how to use that military system for very precise applications by using two GPS receivers at a time rather than the military design for using only one GPS receiver. The problem with a two-receiver (“differential”) solution is that one receiver must be placed at a previously known location that has already been surveyed by the government as a reference point, and that GPS receiver must not move while the other “mobile” unit is moved about during a survey. That one reference point usually needs to have a baby-sitter stay there to guard it from being disturbed, and the reference receiver along with the babysitter present a significant cost to the surveyor. The LSU GULFNet solves that two-receiver problem by providing reference stations throughout the State of Louisiana on a 24/7 basis to Land Surveyors with a data cellphone and a single survey-grade GPS receiver!
  GPS receivers come in a variety of shapes, styles, applications, and prices. Most people are familiar with the units that are for providing directions in vehicles as well as cellphones and shirt-pocket receivers intended for hiking, hunting, and ones integrated with fish finders. Consumer-grade GPS receivers can generally provide positional information that is good to perhaps 20 to 30 feet. Land Surveyors are generally involved with more elaborate receivers designed for high-accuracy applications that can provide precisions on the order of fractions of an inch. Priced at many tens of thousands of dollars, these survey-grade receivers can receive one, two, or more frequencies from a variety of positioning satellites including American GPS satellites, Russian GLONASS satellites, and eventually various other planned satellite providers.

• CAN I USE MY SINGLE FREQUENCY SURVEY GRADE GPS RECEIVERS FOR ELEVATIONS?

  Survey-grade GPS receivers can easily provide horizontal positions (latitude & longitude) to repeatable accuracies within the size of a dime (or even better). However, getting elevations in a reliable manner is quite involved. The accuracy of an elevation obtained with GPS equipment is generally three to seven times less accurate than for a horizontal position. That is due to a number of variables, the primary reason being atmospheric and solar conditions affecting accuracy. Land Surveyors use both single-frequency GPS receivers and dual-frequency GPS receivers for professional applications. The single-frequency (cheaper) GPS receivers are intended for horizontal-only applications because the instruments are not capable of properly compensating for atmospheric and solar conditions (ionospheric effects). Sometimes one can obtain correct values and sometimes one cannot obtain correct values with a single-frequency survey-grade GPS receiver. The problem is that one never knows whether the result is correct or not, and that is why the prudent professional uses the proper equipment for the job at hand.
  Dual-frequency GPS receivers can properly compensate for ionospheric effects if the distance from the reference station is sufficiently close. (Remember, all survey-grade GPS receivers have to work in tandem with another GPS receiver that is in a fixed location and is used as a reference). The further a mobile survey-grade GPS receiver is from a reference station, the greater the accumulation of error for a given amount of time. In other words, if the Land Surveyor goes more than a mile or two from his reference station, for a given level of accuracy needed, the Land Surveyor needs to keep the mobile survey-grade GPS receiver in one place for a longer period of time – from minutes to perhaps hours. The longer an occupation is needed, the more expensive it is to determine the final coordinates or elevation of that point – time is money.

• IS THERE A MORE COST-EFFECTIVE USE FOR GPS SURVEYING?

  Rather than spend hours and hours occupying just a few points and then doing the computer processing afterwards – either that evening or later; Land Surveyors started doing some “real-time” GPS surveying. To perform real-time GPS surveying and to be able to collect data while moving, say on a four-wheeler, in a car, etc.; we term that Real-Time Kinematic GPS surveying, or RTK. The advantage of RTK surveying is that we can get results in real-time, and we can keep moving – that implies increased efficiency and lower cost. However, we are still constrained by distance. The distance from the reference GPS receiver is controlled by how far a radio transmitter on the reference station can maintain a connection with the moving GPS receiver or “rover.” How far depends on radio transmitter power, terrain, buildings, and ionospheric effects. Because the earth is curved and the GPS satellites are constantly moving, if we travel too far from our reference station, there can be different GPS satellites above the sky for the “rover” than there is above the sky for the reference station. When that happens, corrections for ionospheric effects are compromised and RTK accuracy plummets. Work efficiency is compromised too, and we incur greater cost because we have do the work all over again with a shorter distance from our reference station – we need a shorter “baseline.” RTK sounds good, but there are limits because of baseline lengths.