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NGS Control – Class/Order Information
Good morning,
Now that the NGS website is back up, I did some digging, and found where I could download the datasheets for the states I am licensed in, and keep them on my computer. I also downloaded the shapefiles for the control marks.
I also found, and am experimenting with the datasheet utilities that are available, and they work very well, at least from what little time I tried them last night.
My next question is, is there a document that explains the different classes/order of NGS control marks? For example, how much better is a (VERT ORDER-FIRST CLASS II) than a (VERT ORDER-SECOND CLASS 0)? I am also interested in the horizontal values as well.
After this last shutdown, I realized how dependant I had become on that data being readily available, and I don’t want to get caught unprepared again.
Thanks in advance. I probably won’t get a chance to check back in until later this afternoon. Off to the field. Have a good weekend.
Jimmy
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5 Replies
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Kingdom, Phyllum, Class, Order, Family, Genus, Species
That Is all I Remember.
Paul in PA
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The classes for vertical control have to do with the estimated error accumulation along a line. Each class specifies certain procedures, equipment, frequency of calibration and other items like maximum sight length, maximum unbalance, etc.
That said, I consider there not to be a big difference between classes of marks, i.e. first order class I differs little from first order class II or even second order class I, etc, especially in a given area. Where these differences between classes are important, in my opinion, is in long lines, where they were trying to minimize the accumulation of errors.
Since the NGS is not going to relevel the US, or even any subset of it, the existing marks, to me, define the datum. So, if I have a second order line and a first order line in an area, I consider the marks to be basically equivalent in accuracy, and would not distinguish between them for selecting a mark to use based solely on the order and class.
Far more important to me is the STABILITY of a mark. I would much rather use a second order deep rod mark or a mark set in bedrock than a first order concrete post monument, especially in areas of frost heave and other ground disturbances.
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You can find the definitions of the order and classes of control in:
Federal Geodetic Control Subcommittee — Standards and Specifications for Geodetic Control Surveys
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Aloha,
The single, best source of detailed information about the contents of the datasheets is the file dsdata.txt
Every time you retrieve a point from the NGS site you see the following three lines at the top of the page:
The NGS Data Sheet
See file dsdata.txt for more information about the datasheet.
PROGRAM = datasheet95, VERSION = 8.3
The link is in the second line.
Directly it is: http://www.ngs.noaa.gov/cgi-bin/ds_lookup.prl?Item=DSDATA.TXT
Why users do not refer to this file baffles me.
Aloha,
DMM
N21.3
W157.8 -
Aloha,
Unfortunately many data submitters chose to override the stability code associated with the type of monument. They somehow think that their poured in place concrete post is more stable than the default C code. Unfortunately access to bedrock is often limited.
My point is that relying on the stability code rather than reviewing the text of the description detailing setting (e.g. Rod depth) cam mislead.
BTW, NGS does NOT accept differential leveling projects observed to less than second-order. It does have some legacy data classified as third order.
Refer to the file dsdata.txt as described elsewhere in this topic.
Also, NGS order and class are consistent with FGCS standards and specifications. Corps of Engineers orders differ.
Aloha,
DMM
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