4

I asked this question over on the ESRI geonet site, but haven't had any luck there, sorry for the duplication...

I'm working on digitizing a group of maps from the 1950's that show locations of easements along a pipeline. The easement boundaries are recorded with quadrant bearings and distances in feet. (I'm not sure if it's international feet or survey feet, and according to a few sources it shouldn't make much difference? Would love to have someone confirm that though.)

I'm using the Traverse tool to draw the easement parcels, and trying to figure out if I need to do a ground-to-grid correction. The original maps indicate that the basis of bearings is "State Plane Coordinates (Lambert System) California Grid Zone V," with no specific indication of declination or any other corrections. I want the file that I'm creating to be in State Plane CA Grid Zone V (Lambert) as well. So, my first question is: Do I even need to bother with a ground-to-grid correction? And if so, can anyone shed any light on how to find the right correction angle?

Along with this, I'm wondering about keeping everything accurate since I'll need to go from NAD27 (on the original maps) to NAD83 (to play nice with current files). My plan of attack right now is to: 1) create the easement feature class and set the coordinate system to NAD27 in the appropriate SP zone 2) Draw easement polygons 3) Project into NAD83. Would this workflow keep the features accurate? (Or, any suggestions on other/better ways to do it?)

Hope this is clear; dealing with datum/projection details isn't my forte.

Improve this question
3
  • I think your question is formulated so you can probably get someone to help. Just refine (hit edit) with this bit of info. since you want to convert from nad27 to nad83 you are making a correction. Also the declination would be different possibly on each sheet going east or west. Do you have declination printed on original? Also the diff between US and Intl is... US Survey foot 1200/3937 (.3048006096 m/ft and the International Foot (.3048 m/ft exact) ngs.noaa.gov%2Fweb%2Fscience_edu%2Fpresentations_archive%2Ffile…
    – Brad Nesom
    Jun 29, 2015 at 17:41
  • Does the data you digitize need to align to any existing data? Do the descriptions reference offsets, actual calls with bearings, or a mix? Are their PoBs absolute state plane coordinates, or do they reference PLSS section corners? In my experience, if you're trying to match existing data you may as well forget any gtg correction. They'll give you a basis and you get a correction angle from that only to find another line has a different angle difference. Plus, from the 50's calls likely aren't to be too precise in terms of angles (no seconds, or nearest half minute).
    – Chris W
    Jul 2, 2015 at 19:40
  • The descriptions/maps are likely to be a reference or starting point, and you will have to adjust everything to fit on a case-by-case basis, including altering calls both for closure or data matching. I would probably start by digitizing everything in NAD27 per the maps, then project it to NAD83 in a new file and begin modifying that to fit. That way you have the original, as described shapes for record, and then useable, matching data. 'Accurate' is relative as you quickly find when getting into survey related work, and you have to make allowances to match/fit data together.
    – Chris W
    Jul 2, 2015 at 19:47

2 Answers 2

Reset to default
2

If you are working from land survey plats, the basis of bearing, even though it is referenced to California Grid Zone 5, should have a starting, or reference line for the basis of bearing.

This should be at the beginning of the description for the easement.

It may start at a section corner, or some other aliquot corner, and describe the traverse to the easement along a section line, or a distance along a section line, then a perpendicular distance to the beginning of the easement.

If it is simply drawn with record calls, you will have to use them, and find corresponding locations on your background data. (Background data could be an aerial image, PLSS data, GIS data from another source, or anything that you may be using to overlay your data on.)

If you need to incorporate a ground to grid correction for the bearings, take two points from the recorded data with bearing and distance calls, and the same two points from your background data, then do a coordinate inverse between the points from your background data to derive azimuth, and distance for the two points. The greater the distance between the points being compared, the better.

You can then compare this data with what is on your plats, and apply the difference in bearings to the traverse you are computing, you may also find discrepancies with the recorded distance, and the inverse distance. At this time, the difference in bearings is more important unless the distance discrepancies are really large. (Greater than a few feet). It would be a good idea to find multiple coincidental locations to check this with. This assumes that you are able to relate the plats to discernible locations on the ground, and your background data.

Working up the traverse in NAD27, then converting to NAD83 should not be an issue. Just make certain that you use the proper transformation methods. I realize that nearly everyone uses the NADCON transforms, but every now and then you find someone who uses something else. It is not common to do so.

Improve this answer
2

I am not from California but in my state the declination should be listed on the plat or record of instrument, at least it usually is. If not I believe this may help, put in the area the survey was done and the date it was done and it should give you declination. NGS has alot of cool tools btw

http://www.ngdc.noaa.gov/geomag-web/

The difference between survey feet and intl feet is like 2 feet per million meet or something to that effect, so not very large if you are dealing with a small area but it can add up over a long distance.

Your work flow seems on step to me, but I am probably not the best to ask, hah

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.