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Mapping guilford, rev


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Mapping guilford, rev

  1. 1. 2011 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. G. Gunn Guilford County 4/21/2011
  2. 2. First ThoughtsMemo re: Parcel Mapping ControlTo: Ben Chavis, Greg French, Assistant Tax Directorscc: Stephen Dew, GIS Administratorfrom: Greg Gunn, Mapping TechnicianBen and Greg, these notes are primarily addressed to you, in response to your requests for suggestions on how wemight improve our methods, but I thought that Stephen may have suggestions or comments and may already beworking on some of these issues, such as R/W. At any rate, since parcel mapping is a subset of our GIS and Stephenis the expert and administrator of the overall system, he would have to implement any new components, so his inputand guidance are essential.Following our discussions over the past few weeks and my limited experience over the last three months, I’ve beengiving quite a bit of thought to methods we can use to establish control for our parcel mapping. As of now, there areno control measures that I can discern. The result is a parcel layer that seems to shift around arbitrarily over time andspace with its attendant problems, compounded by questionable or obviously errant data inherited from old papermaps. We should have some kind of framework or skeleton on which to flesh out our map. I know that in the pastour parcel mapping staff was overwhelmed by the huge volume of recorded documents. Expediency was of primaryconcern, what with deadlines, etc. While we are relatively caught up, given the current economic downturn, thereare methods we can implement to start improving quality. It may take several years, or more likely would be acontinuous process, but by following the methods outlined below, I think we can begin incrementally improving ourparcel data.Let me know your thoughts. Are these suggestions feasible, practical? Are there issues of which I’m simply ignorantor 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 POBs (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 POBs 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. 3. 3. Remap adjoining parcels from these control polygons outwards to logical bounding areas, e.g. blocks, R/Ws, 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. (Arent 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 arent 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 (POBs) o Control polygons (parcels, subdivisions) o Control areas (blocks)
  4. 4. Additional Thoughts on Mapping Control: Policy and Procedure orMethodology versus Madness!Why we need a policy:NC Mapping Specifications require that property corners tied to grid be mapped and labeled accordingly( ) In fact, the entire Land Recordsmanagement program for all one hundred counties in North Carolina is based on the NC State PlaneCoordinate System. It is the basic framework for all of our spatial data! Land Surveyors in NC arerequired by law (see NC GS47-30 and GS 102) to tie boundary surveys to grid when a property lieswithin 2000 feet of a monument.One of the reasons these requirements have been legislated is so that we can utilize them to improve theaccuracy 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 ultimatelyto the citizens and taxpayers whom we serve.It is imperative that we impose some standards, provide some guidelines and develop a methodology - anenforceable, mandatory policy to begin taking advantage of these resources when available, and whenthey are not, to employ other proven methods to continuously improve our cadastral mapping. Everyonemust be on board. We cannot be inconsistent, nor can we continue to use sloppy techniques to shoehornnew parcels into poorly mapped areas of our existing parcel fabric.By implementing a policy based on sound methodology we can incrementally, consistently andcontinuously improve our cadastral map products and those based on them. If we continue to letexpediency 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 arethe only way to improve; and as we improve, over time, mapping will become easier. However, right nowwe do have a considerable amount of work ahead of us. This is something we should have begun decadesago, but it’s never too late to start upholding the mapping standards specified by the State of NorthCarolina 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. ( ) 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,
  5. 5. by traversing in property descriptions by metes and bounds we are able to evaluate the quality of the surveys and weight them accordingly against adjoining surveys. Heads up digitizing when geo-referencing to bad data is only perpetuating, compounding, and creating more problems! 5. Identify priority areas for immediate improvement efforts by whatever criteria are deemed most important. The worst areas by a general visible inspection of GIS? Areas that exhibit poor occupation, R/W not aligned with roadway, property lines running through houses, etc. Areas of special interest? Those known to be under development or most likely to be in the near future. Input from the assessors and the Planning Department might be helpful here. Areas where roadway or utilities improvement projects are planned or in progress, large subdivisions, etc. Special use areas? Haw River State Park, Greenway Belt around Greensboro?Greg GunnApril, 2010
  6. 6. Using Grid Ties for Property Mapping See PB 178-118. This is an excellent example; a good survey which illustrates nearly all of the steps that one might find necessary to place a survey on the map by a grid tie.Let’s look at the Grid Tie Detail shown in the upper right corner of the plat: not every plat will have adetail inset like this one. Sometimes the grid tie is shown in the main body of the plat, but the essentialsare the same: The monuments used for the tie, along with their NC State Plane Coordinates in either U.S. Survey feet or meters, and the datum used - in this case, NAD 83(86). More about datums later. The bearings and distances from one monument to the next, and thence to a property corner. Often, the coordinates of the property corner tied will be shown also, as they are on this plat. Look at the southwest corner of the property. With these coordinates alone we can place this property corner on the map right where it needs to be. However, the rotation will be incorrect after traversing in the boundary. Look carefully at the Grid Tie Detail again and you’ll see that there are two sets of bearings for the tie from monument Cone to the property corner – a map bearing and a grid bearing.Now, look at the north arrow on the plat and you’ll see why. The surveyor’s basis of bearings is PlatBook 60, page 120, the plat that originally created the two lots being recombined by our new plat. So, weneed the grid bearing and the plat, or map, bearing in order to rotate the boundary in correctly. We’ll getto that in a few minutes, but first we have to convert from meters to U.S. Survey feet.
  7. 7. We won’t always have to do the conversion. Often the surveyor will have already done so for us, but it’svery important to recognize the difference and to do the conversion correctly when necessary. If we didn’tdo the conversion for this plat, the property would end up about 260 miles southwest of its actualposition, somewhere much closer to Gainesville, Georgia, than Greensboro, North Carolina!And it should be noted that the U.S. Survey foot, the official conversion from meters used in NorthCarolina, is different than the International foot used in some states, such as South Carolina. The officialconversion we use is:1 meter = 3.2808333333… U.S. Survey feetIf we use the conversion for International feet we’ll be off about four feet. And, it’s very important touse at least nine significant figures – that’s nine figures after the decimal place - or huge errors will beintroduced given the large numbers we’re using.Consider that typical values for NC State Plane coordinates, in feet, in Guilford County range fromroughly 782,000 to 914,000 for Northing; and 1,690,000 to 1,841,000 for easting. Converting thesenumbers to meters, that’s 238,354 to 278,588 for northing and 515,113 to 561,138 in easting (rounded tothe nearest meter.) I used twelve significant figures for my conversion. That’s all my calculator will hold,but I wanted the conversion as accurate as I could get it just to make a point:Try converting the lower range of numbers in meters for northing and easting shown above back to U.S.Survey feet, to the nearest foot, using only two significant figures in the multiplier: 238,354 meters ×3.28 U.S. Survey feet/meter = 781,801 U.S. Survey feet 515,113 meters ×3.28 U.S. Survey feet/meter = 1,689,571 U.S. Survey feetNow, compare the results with the lower range of numbers in U.S. Survey feet we started with forGuilford County: 781,801 ≠ 782,000 1,689,571 ≠ 1,690,000We have a difference of almost 200 hundred feet in northing and nearly 430 feet in easting! Horizontally,by the Pythagorean Theorem, that’s a linear distance of over 474 feet! I hope this illustrates theimportance of using at least nine significant figures!
  8. 8. Okay, now let’s do the conversions for our plat:NCGS Monuments:SUMMIT:N (y) = 262,735.496 meters = 861,991.373 ft.E (x) = 541,609.989 meters = 1,776,932.106 ft.CONE:N (y) = 262,356.785 meters = 860,748.886 ft.E (x) = 541,452.174 meters = 1,776,414.341 ft.Iron Pipe Found:N (y) = 262,423.141 meters = 860,966.588 ft.E (x) = 541,659.416 meters = 1,777,094.267 ft.Next, we’ll traverse in the grid tie using the beginning and ending coordinates to get a closure. I alwayscalculate and adjust the closure because it serves to check the integrity of the survey and insures that Ididn’t make an error keying in the data.First, select the traverse tool, make sure Closed Loop is unchecked, and click Edit after the Start: field inthe Traverse dialogue box. Then enter the coordinates of the first monument. Important: Note thatArcEditor uses x and y, not northing and easting. The easting goes in the x field and the northinggoes in the y field!
  9. 9. Then do the same for the End: field using the coordinates of the Iron Pipe Found, and traverse in the callsfor the grid ties. Be careful! The grid distance on this plat is in meters and the grid distance is what weuse, but in feet. So we’re going to have to do some more conversions, but take heart – we won’t have todo this on every grid tie we encounter. As I said before, I chose this example because it illustrates nearlyevery problem we are likely to encounter.Once all the calls are entered, click on the closure button:Looks good to me! (I have to confess that initially I mistyped the distance in the first call, using 1346.510instead of 1346.051. That gave me an error ration of 1:4513, prompting me to check my figures again –another good reason to check closure!)
  10. 10. Next, click the Adjust button and accept the results. Now we can traverse in the map tie and the boundary.Snap to the beginning of the line leading to the tied property corner and traverse in the map bearing andgrid distance. Again, we want grid distance, not ground, to put our point of beginning back at thecorrect grid coordinates after we rotate.Of course this time we aren’t checking closure – there’s nothing to close on here – so we must be extracareful that we enter the map bearing correctly. Now we have two lines from NCGS monument Cone –the correct grid tie and a second line which will be used to rotate the boundary back to grid after we’vefinished traversing it in. We must be careful that we start traversing the boundary from the correct line –the one with the map bearing!
  11. 11. Alright! We’ve finished the traverse and got a killer closure thanks to a good survey!
  12. 12. So, here’s what we have so far:And a close up on the map tie versus grid tie:
  13. 13. Now we need to rotate the boundary and map tie to the grid tie. Just hold down the shift key and select themap tie:
  14. 14. We need to rotate everything selected from the monument Cone. In order to do that, we need to snap therotation point to the end of the line at the monument.And, oh look! I have a layer that shows NCGS monuments with their NC State Plane NAD 83(86)coordinates! Where’d that come from?
  15. 15. Once we’ve snapped the rotation point to the right place, we can zoom in to the other end of the grid tieand start rotating:
  16. 16. I zoom all the way to 1:1 to get the rotation as tight as possible:
  17. 17. Finally:It may seem like overkill to zoom in that tight, but my personal opinion is that once we’ve made the effortwe should go all the way and get it as precise as we are able. This is especially important when remappinglarge areas based on one good grid tie for control. If we get lazy and our rotation is off a foot from theactual tie, it begins to add up.
  18. 18. And here’s our grid tie precisely located on the map:I’ll have more to say about datums, NCGS resources and some common surveying/drafting errors to lookout for in another installment. But first, look at the next few pages for an example of a large area cleanedup by using a grid tie:
  19. 19. Future Topics:• Datums and the importance of recognizing the huge difference between NAD 27 and NAD 83, how to do the conversion, and the relatively minor differences in iterations of NAD 83.• Common mistakes with grid ties seen on plats and how to resolve them.• NCGS Database and Tools.• Grid ties without monuments - OPUS, RTK, and VRS.• Establishing a good P.O.B. and rotation without a grid tie - building Right of Way off the imagery, and using other common ties shown on plats such as to intersections and adjoining properties’ corners.• Remapping large areas - establishing control; weighting one survey against another; selecting a common corner from an adjoining deed and rotating in; systematically cleaning up line work as we go; etc.• Best practices for boundary resolution from a cadastral mapping perspective.• Getting the most out of ArcEditor’s sketch tools and COGO tools.• Quality control/quality assurance and the importance of following the standards.
  20. 20. Determining a Good Point of Beginning by Building Right of WayWhen grid ties aren’t available in an area that needs remapping, building the road right of way at anintersection is the next best thing. Roads are easily seen on the imagery and they are at least semi-permanent. Roads shown on platted subdivisions don’t usually move around much, though there areexceptions, especially on older plats; but for the most part we can be reasonably sure of getting a goodP.O.B. by reproducing the right of way shown on a plat.To demonstrate, take a look at REID 0166041, lot 69 of PB 121-54.
  21. 21. Clearly, this area needs some work. Most of the road is not in the R/W as we now have it shown. Mostlikely, this subdivision has not been touched since it was digitized from the old paper maps, but we’regoing to fix it. In order to do that, we must carefully trace the centerline of the road and use the copyparallel command in ArcEditor to build the R/W.The first thing we’re going to do is turn off the Factory Parcel layer so we can see the road better. Thenwe’ll carefully trace the centerline. It’s much easier to judge the centerline when the pavement is marked,but in this case we have plain asphalt. We can zoom in and eyeball it on one end, then zoom in on theother end and do the same. If it’s hard to judge, you can zoom in and draw a line from EP (edge ofpavement) to EP on each end, then snap a line to the midpoint of those lines by right clicking on the linewith the pencil tool, then choosing snap to feature/midpoint, as shown below:Next, select your centerline and use the copy parallel command from the Editor menu. The plat shows a50 ft. R/W for both streets, so we can select both centerlines and copy them at the same time. Rememberto enter half the R/W distance (25 + 25 = 50) in the copy parallel dialogue box that pops up, and chooseBoth, as shown in the illustration below:
  22. 22. And here’s the result:
  23. 23. After deleting the centerlines, and trimming and splitting the R/W lines we have this:But looking at the plat, we see that there are curves at the intersection, not straight lines. In order to makethose curves match the R/W we have to copy parallel those lines to get the radius point. Looking at theplat we see that the radius at the intersection is 25 ft. So we grab the lines and copy parallel 25 ft. again,the radius distance. Notice I’ve only chosen one set of lines on the west side of the intersection this timeand I’m only copying to the left. When using this command there are pointers showing which way thelines being copied are oriented so you’ll know which side to choose, as shown below.
  24. 24. After copying all the R/W lines in the right direction, the radius points are found where the new linesintersect:
  25. 25. Next, we trim the lines as shown here:In order to get the PC (point of curvature, or beginning of the curve) and the PT (point of tangency, orend) of the curve, we snap lines from the radius points to the R/W lines. The radius point is alwaysperpendicular to the tangent sections, so it’s important to snap to the radius point, then right click on theR/W line and choose perpendicular from the contextual window, then right click again and choose snapto feature/ edge. Even if one of your R/W lines coming into the curve at the intersection is a curve itself (acompound curve), ArcEditor is smart enough to find the PCC (point of compound curvature) or the PRC(point of reverse curve.) When choosing perpendicular to a curve, the line is drawn on a radial, a pointthrough the intersection radius that lines up with the radius of the other curve coming into it.See the illustration on the next page for clarification:
  26. 26. If you’re attentive, you’ll see that I mislabeled this diagram. That’s actually a PRC, not a PCC, becausethe curve at the intersection reverses direction. The curve approaching the intersection from the northwestis to the right; the one turning northeast is to the left.
  27. 27. Here’s another one illustrating the importance of snapping the lines from the radius point perpendicular tothe tangent section. It’s far more apparent when the intersection is at an angle other than 90°. The platshows a 60 ft. R/W for both roads with a 20 ft. radius at the intersection. Had we simply extended the 20ft. offsets (from the radius point) to the R/W instead of snapping them perpendicular, we would haveplaced the PT too far to the southeast and the PCC too far to the southwest. I began traversing this onefrom the PT and then rotated to the tangent running northwest. Look how well it fits:
  28. 28. Let’s look at the first plat again, PB 121-54. I’ve started traversing in the section west of Wild Fern Driveand north of Case Ridge Drive with a P.O.B. at the PC of C26 as shown on the plat:After traversing in the entire plat, we can rotate the line work from the P.O.B to match the R/W built fromthe center line. Just click the rotation tool and drag the insertion to the P.O.B. and zoom in tight to the endof one of the R/W lines. We need to use some judgment at this point – a long straight tangent section isbetter to rotate in on than a small curve. Likewise, a paved road that is clearly shown on the imagery isbetter than an unpaved, winding road with edges that are partly obscured by shadows.The next page shows rotation to the tangent section running west. It would have been better to build theR/W along that section much longer than the one in this demonstration, but as you can see from the finalresults, this section of the subdivision looks much better now.
  29. 29. First, the insertion point has been dragged to the P.O.B. (snapping set to end and vertex of theconstruction lines:
  30. 30. Here’s the final result after rotating and cleaning up the lines used to construct the R/W, showing the newlines on top of the old polygons:And on the next page, without the polygons:
  31. 31. That’s much better! The R/W is centered on the roads and there are no property lines running throughhouses.
  32. 32. A Few Words About Rotation:You may be wondering why we have to rotate most of the surveys we map. Most of the time a surveyorwill start out with north defined arbitrarily, and then rotate in on the bearing for the longest property linehe measured, using the bearing for that line as shown in an older survey. The older survey was probablybased on the magnetic bearing established for that line. There is a difference between magnetic north andtrue north, which is called magnetic declination. There is also a difference between true north and gridnorth for that matter. To further complicate things, magnetic north varies by location on the earth, andalso over time! For instance, magnetic north at any given location will be different today than it was in1950 or 1980, etc.The only surveys we don’t want to rotate are those that are tied to grid and show grid north as their basisof bearings. Sometimes a surveyor will tie to grid, and then rotate to an old plat bearing. Usually they willshow the grid bearing and the map bearing for the actual tie from the monuments so we can rotate to grid.And here’s a hint: NCDOT has been basing all of their surveys for R/W on grid for several years now.Any NCDOT R/W deed recorded in the last ten years or so should not be rotated!G. Gunn6/21/12