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Marks Presumed Destroyed


TerraVador
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I believe this is a question only the NGS can answer, but if any of our regulars know, chime in. I know you can get a Datasheet for a mark that is reported destroyed, COND=X. But what about a Vertical Mark that is presumed destroyed, nonpub code COND = Z? What can I find out about one of these? I am guessing that COND=Z will block Datasheet retrieval, while COND=X will not. I looked thru the entire list of NONPUB marks for the state of Texas which you can download in a single file under Misc. Files in the Yearly Archives, and not one that I could find was listed as COND=Z. I am guessing that COND=Z will cause the database to return COND=D instead, even if this is not true.

 

The reason I am asking is I believe I located a presumed destroyed mark, but can't be sure without the descriptive text. I don't think I can get the descriptive text because the mark is presumed destroyed, kinda of Catch-22 (or a Catch COND = Z).

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Terra,

 

Sorry it has taken so long to get an answer to you on this, but I think I can help you in part, and for the rest, I would recommend writing cheryl.malone@noaa.gov for the things that only she can ascertain. NGS has a number of ways they can use their Database which we have no access to, so searching for Vertical survey with Non Pub Codes of Z may be something she can do.

 

Going forth, there are factors and difficulties involved and here are a few of what I think they are.

 

First, the "Z" Non Pub code only applies to vertical control. It is not used with horizontal control, and this narrows the field of it's use. Vertical Control was historically derived from a survey process called Differential Leveling. you can easily google this is you want to know more, but in short it is a redundant process of following a path starting somewhere and leading to somewhere else and measurements are carefully taken with a Leveling Rod held by the Rod man, rested usually upon a survey marker and in rarer occasions an object determined to be of adequate stability, such as a Rivet in a Bridge or the Top Bolt of a Fire Hydrant. Then the Rod was read with a Survey Level mounted on a Tripod by the instrument man.

 

Then the sum or difference is taken from the previous location to determine the elevation at the new location. The process is repeated many times along the same path to confirm and tighten the overall accuracy, and then all of the measurements are statistically use in further calculations to enhance the accuracy over an entire network of measurements.

 

Now the "Z" code is presumed Destroyed. The inherent problem that follows this especially in leveling, or Bench Marks, Vertical control, is that for the most part, the most common device used to mark a survey station for vertical work was a Brass Disc Monumentation. NGS has very strict rules they adhere to for what can be used or adopted for use as a "Bench Mark", and they are equally strict over classifying these stations destroyed as well, and so most that have gone missing in actuality are classified as not found. So you see, finding benchmarks, make that Vertical Control Marks, that were actually submitted to NGS as destroyed that passed their criteria for destroyed marks are probably the rarest of them all, because most were survey markers in the first place.

 

You will find a lot more destroyed stations in the horizontal plane, as there were a lot more objects considered usable for triangulation as it was optical and used trig to determine locations, and they did not have to be physically touched to measure them. So we have steeples and stacks and towers... All things you cannot use for differential leveling. NGS has a more lenient standard for considering "Landmark Style" Triangulation Stations destroyed. The objects used for vertical control stations were no where near as diverse as those used for Horizontal control. To be sure, Most of vertical control really are marked as pure survey markers, as opposed to adopted objects.

 

So Here we are. What you are looking for is pretty rare. Perhaps NGS can help you search for these in your locale. But please remember this, if it is destroyed in their opinion, it means they were furnished empirical evidence as proof of destruction. So if you can find something, it will be super tough, because it has already been proved to NGS's satisfaction as really destroyed, and they have seen that it really is. It won't be a landmark so rule those out. Maybe a specific spot on alandmark, but if the landmark is gone, so goes the station.

 

I would try providing NGS with whatever information you can about the station you have in question when you inquire, most importantly the Station location in Latitude and Longitude, in Degrees Minutes and Seconds. Ask them if they have any records of a station feel free to add why you are curious, and anything else that you feel will help them help you.

 

Good Luck on this one, It will be pretty challenging.

 

Rob

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