Okay, here's a serious question for the GIS/GCDB experts:
The BLM “PID” (Point identification ID) used by the GCDB system is explained in the link below:
Okay FINE! It's a SIX-digit code that works dandy when there is ONLY ONE [say] Northeast Corner of Section 10. What the linked document FAILS to address, is situations where there are TWO (or MORE) Northeast Corners of Section 10.
YEAH...I know...that isn't supposed to happen, but it DOES!
We have been asked to furnish these BLM-PIDs on an ongoing project that contains a LOT of these double corners (about a Townships worth).
Both of these Monuments/Corners evidence Approved GLO Surveys (1850s & 1880s/90s), and BOTH of these corners CONTROL Federal Patents, and BOTH of these Corners have been used historically to build fences, convey land, etc. etc. In other words, THEY have BOTH been relied upon at one point or another, and in some cases BOTH still are (but that's a whole 'nuther' problem).
So in the case of the NE Cor. of Section 10, the BLM-PID would be 500600. Well what about that “other” NE Corner of Section 10 (situate some 479 ft. North, & 249 ft. East of the Original Corner)????
507604 (proportioned PID)????
I dunno...I like 500600(a) & 500600(b) but that ISN'T SIX digits!
Any ideas?
Loyal
Don't they usually number witness corners 1 and 2, etc? I would consider that.
That document is not the definitive document on the system.
I can try to track down other sources, but as my memory serves me 'offset corners' which abound in the PLSS, particularly along township exteriors, but also occur a lot interior to townships are designated with id's 2 or 3 offset from the normal
So a CC offset from 300700 might be 302700 or 303700. I think I recall that 4 or 6 offsets are used for WC's or MC's, but I need to look at the "official" documentation to be sure.
Along exteriors generally the ID which applies to the regular control for that township is used, so that there may be a different ID in the adjacent township on occasion since this ID scheme is township based.
Software had to learn to identify with those ID differences along the township boundaries and develope alias tables or match tables (.MAT files) to tag them.
I will see what I can find on line.
- jlw
Reply to self...
Looking at this source, which looks more like our original GMM documenation:
http://www.docstoc.com/docs/2141340/GCDB-User-Guide
It looks like 1 ro 2 digit offsets were more used on interior lines to a township and the 3 digit offsets on the exterior. There is a simple table which defines this protocol in the GMM docs and one would think you could find it on line somewhere.
- jlw
Thanks Jerry (and Traci too).
Jerry, I was hoping that you would chime in here, you know more about this kind of stuff than ANYBODY I have been able to talk to so far.
This "offset" Section on top of (but OFFSET from) another Section, is a real PITA. They are BOTH Section 10 (and several dozen similar sections) BUT represent DIFFERENT Approved GLO Surveys. In some cases (so far) we have a full set of Eight Corners (MONUMENTS) on BOTH Section 10s (offsets vary a little, but not much).
Loyal
Yes, id designations can be a PITA and a challenge.
Here is a more complete set of docs and on page 60 in Appendix A it shows what I am talking about as the basic standard:
http://web.nmsu.edu/~kwurm/software/wg/WGTechRef.pdf
As it states offset ids using 7 8 and 9 are also allowed to designate the plethora of such situations.
It is generally not abolutely critical as long as they are in a logical order.
The 0's are normal corners, the 40's would normally be 1/4 corners, the 20's and 60's 1/16ths, the 10 offsets 1/64ths and the 5 offsets 1/256's for "normal aliquot corners.
If you have to go that far for offsets or other bizarre corners it would not normally be fatal.
You have to do what you have to do.
- jerry
Thanks again Jerry, I'll forward that along (after I have read it).
Loyal
If the hexadecimal, instead of the decimal, number system had been used, it would be possible to refine positions with 60% greater resolution. Since all of this process is a "code" and requires deciphering algorithms, hexadecimal being based on the decimal number 16, would have been much more practical than the decimal system. Binary is good for computers, decimal is good for people with 10 fingers and hexadecimal is good for section subdivision into halves, quarters, sixteenths, etc.
This is one way to attain the degree of precision you'd like and still keep it to six digits.
