Mapping Guilford County by Grid Getting the most out of the NC State Plane Coordinate System This is an ongoing series of memos and articles discussing the necessity of implementing quality control/quality assurance for Cadastral mapping in Guilford County, North Carolina, and how to go about it. 2011 G. Gunn Guilford County 4/21/2011
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Mapping Guilford
County by Grid Getting the most out of the NC State Plane
Coordinate System
This is an ongoing series of memos and articles discussing the necessity of implementing
quality control/quality assurance for Cadastral mapping in Guilford County, North Carolina,
and how to go about it.
2011
G. Gunn
Guilford County
4/21/2011
First Thoughts
Memo re: Parcel Mapping Control
To: Ben Chavis, Greg French, Assistant Tax Directors
cc: Stephen Dew, GIS Administrator
from: Greg Gunn, Mapping Technician
Ben and Greg, these notes are primarily addressed to you, in response to your requests for suggestions on how we
might improve our methods, but I thought that Stephen may have suggestions or comments and may already be
working on some of these issues, such as R/W. At any rate, since parcel mapping is a subset of our GIS and Stephen
is the expert and administrator of the overall system, he would have to implement any new components, so his input
and guidance are essential.
Following our discussions over the past few weeks and my limited experience over the last three months, I’ve been
giving quite a bit of thought to methods we can use to establish control for our parcel mapping. As of now, there are
no control measures that I can discern. The result is a parcel layer that seems to shift around arbitrarily over time and
space with its attendant problems, compounded by questionable or obviously errant data inherited from old paper
maps. We should have some kind of framework or skeleton on which to flesh out our map. I know that in the past
our parcel mapping staff was overwhelmed by the huge volume of recorded documents. Expediency was of primary
concern, what with deadlines, etc. While we are relatively caught up, given the current economic downturn, there
are methods we can implement to start improving quality. It may take several years, or more likely would be a
continuous process, but by following the methods outlined below, I think we can begin incrementally improving our
parcel data.
Let me know your thoughts. Are these suggestions feasible, practical? Are there issues of which I’m simply ignorant
or just haven’t considered? Do you have any further suggestions? Thank you for considering the following points:
1. Establish, maintain and preserve a control grid based on POB's (point of beginning) with NC State Plane
coordinates from plats with grid ties. All new plats with grid ties should be traversed in by metes and
bounds from the POB. "Grid tie" should be entered in comments, or a new field added to the attribute table,
a simple yes/no field would do. Our policy/guidelines should require that all new recorded plats be
examined for grid ties and their POB's on grid entered in a data base. It can be very simple: northing,
easting, plat book and page number (y, x, ref )
2. Preserve these as control points, and the metes and bounds derived boundaries as control polygons, a
simple geo-database, preserved, maintained, protected, and locked down on a separate layer with restricted
editing access.
3. Remap adjoining parcels from these control polygons outwards to logical bounding areas, e.g. blocks,
R/W's, rivers or other natural boundaries, etc. and document each parcel with deed and plat references. Map
subdivisions by metes and bounds, check closure and balance traverse, or force closure when description is
ambiguous, vague, or absent and justified by logic or geometry. (Aren't those the same thing?)
Rotate/translate to best fit, evaluate, adjust, project lines, perform distance/distance intersections, etc. Make
some effort to document decisions so that boundary determination from the source documents is traceable,
reproducible, validated. Can each mapping technician be given a separate layer similar to the traverse layer
that we can use to keep our interpretation, documentation for these bounding areas? Or a separate database
with hyperlinks?
4. When a bounding area is completed and has been deemed worthy (establish some criteria, metrics,
judgment,) it should be saved as a control area, and locked down, or at least preserved until subsequent
revisions have been evaluated.
5. If there is enough time/manpower, go backwards through the plat books and record each POB with grid ties
in the database (y, x, ref) and map their control polygons.
6. Step back and look at the big map! Control points can be done far more quickly than their corresponding
and dependent polygons. If we could map just the points, when problem areas pop up we can quickly check
for nearby control points and build their corresponding polygons when justified. This would have the added
advantage that when we are caught up or ahead of schedule, we can quickly identify control area gaps and
start filling them in.
7. Where grid ties aren't available, find other ties in a block or bounding area, e.g. centerlines of intersections,
R/W, etc., build the block/area, and document the work.
8. Speaking of R/W, get some. Establish good R/W for roads and maintain and preserve it. Map all NCDOT
projects tied to grid. CAD files (Microstation DGN) are available, and possibly shape files. (Does anyone
with the County have Microstation or Land Desktop/Autocad?) Or we could traverse road alignments from
NCDOT grid ties and offset to build the R/W.
9. Parcel boundary determination by deeds and plats should be preserved separately from R/W, especially
when R/W is taken after a platted subdivision, when R/W evidence is missing or otherwise ambiguous, e.g.
deed calls to center of road or only short chords given instead of curve data. R/W layer should cut and
cover boundary layer on GIS, but parcel boundary preserved underneath if possible.
10. Control hierarchy:
o Monuments
o Control points (POB's)
o Control polygons (parcels, subdivisions)
o Control areas (blocks)
Additional Thoughts on Mapping Control: Policy and Procedure or
Methodology versus Madness!
Why we need a policy:
NC Mapping Specifications require that property corners tied to grid be mapped and labeled accordingly
(http://www.ncpropertymappers.org/specifications/index.html ) In fact, the entire Land Records
management program for all one hundred counties in North Carolina is based on the NC State Plane
Coordinate System. It is the basic framework for all of our spatial data! Land Surveyors in NC are
required by law (see NC GS47-30 and GS 102) to tie boundary surveys to grid when a property lies
within 2000 feet of a monument.
One of the reasons these requirements have been legislated is so that we can utilize them to improve the
accuracy of our maps! If we do not use them, we are wasting an extremely valuable resource; moreover,
we are failing to meet our responsibilities to the NC Land Records Management Division and ultimately
to the citizens and taxpayers whom we serve.
It is imperative that we impose some standards, provide some guidelines and develop a methodology - an
enforceable, mandatory policy to begin taking advantage of these resources when available, and when
they are not, to employ other proven methods to continuously improve our cadastral mapping. Everyone
must be on board. We cannot be inconsistent, nor can we continue to use sloppy techniques to shoehorn
new parcels into poorly mapped areas of our existing parcel fabric.
By implementing a policy based on sound methodology we can incrementally, consistently and
continuously improve our cadastral map products and those based on them. If we continue to let
expediency be our only guiding principle we will continue to produce and perpetuate substandard maps.
It will take a concerted effort and a considerable amount of time, but sound and consistent methods are
the only way to improve; and as we improve, over time, mapping will become easier. However, right now
we do have a considerable amount of work ahead of us. This is something we should have begun decades
ago, but it’s never too late to start upholding the mapping standards specified by the State of North
Carolina and deserved by its citizens!
Additional thoughts on methodology:
1. Grid ties! Whenever and wherever available. No exceptions! Everything recorded from this day
forward, and going back to older documents when time and resources permit.
2. In the absence of grid ties, build right of way from careful tracing of roadway centerlines and
offsets, then start mapping from the intersections of R/W’s. Even better, obtain NCDOT roadway
plans when available. All NCDOT projects from the last ten years or so are tied to grid! Getting
information from the NCDOT has been difficult in the past, but partly due to efforts of the
NCPMA, that may be changing. (http://www.ncpropertymappers.org/dot.html )
3. Utilize surveyors’ ties to intersections and adjoining property corners when available. Always
strive to find a good point of beginning by whatever means available when beginning to map a
document.
4. COGO (traverse by coordinate geometry) everything! No more heads up digitizing! We cannot
continue to geo-reference and rubber sheet plats to a poorly mapped parcel layer! Additionally,