A common method to test your field gear is to check results against published values on passive control marks. While there are caveats to doing this (more on this later), it is a practice that can be quite valuable.

There are many National Geodetic Survey horizontal and vertical control marks, and you can find datasheets for them through their interactive map and text lookup: geodesy.noaa.gov/datasheets/

Several cities and parishes around the state maintain sets of control records.

Keep in mind that when you are comparing an observed position to a published position, you have to make sure you are comparing apples-to-apples. When using passive control, you need to consider the source, method of establishment, reference framework (datum), and especially the date. For example, there are control marks that may have multiple published values, and none of them are “wrong”. A mark might have an NGS value, a local parish value, etc. They are all valid for the specific folks who published that value. However, you need to consider what datum and year the values were established. Due to subsidence and plate tectonics, the value may have changed over 20 years and there have been successive reference framework (datum) updates.

C4Gnet.XYZ constrains all stations to the National Geodetic Survey National Spatial Reference System (NSRS) to establish active control in Louisiana. When you use C4Gnet.XYZ, you get a position in the NAD83-2011 Epoch 2010.00 reference framework, the same as if you did an NGS OPUS solution. Is the control record expressed in the same reference framework? Likely it has but look at the date of the record and consider how much movement has happened in the intervening years.

Establishing your own check point can be very useful. Some firms and public entities set a point, for instance, in their parking lot. It comes in handy for seeing if all your gear is working before you head to the field. And to see if all the settings are Ok and you are getting the values on the reference framework you are expecting. There are several common ways folks establish coordinates on their own check points. You could do several days of long static and submit to NGS OPUS. Or do several hours of static and use the WSRN online post-processing tool for several hours. But also keep in mind that (depending on where you are in the state), the plate velocities can change values even after just a year.

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