Subdivision Mapping: Making the Pieces Fit P David P. Thaler, GIS Analyst A geographit White Paper, June 2009 1525 Oregon Pike, Suite 202 Lancaster, PA USA 17601-7300 Phone: 717-399-7007 Fax: 717-399-7015 TUwww.geographIT.comUT
Table of Contents Introduction... 1 Case Study: Chester County, Pennsylvania... 2 NEW APPROACH TO SUBDIVISION MAPPING... 2 Using GPS in the Subdivision Mapping Process...3 Selecting and Mapping Property Markers...4 Fitting it all Together... 6 Additional Benefits... 7 TIME SAVINGS AND PUBLIC PERCEPTION... 7 Conclusion... 7 Subject Matter Expertise... 8 List of Figures Figure 1 Subdivision Misalignment Issue...1 Figure 2 Trimble GPS Equipment...4 Figure 3 Candidate Property Markers on a Subdivision Plan...5 Figure 4 Different Types of Property Markers...5 Figure 5 Geo-referenced Subdivision Using GPS Control Points...6 List of Tables Table 1 - Comparison of Old and New Subdivision Mapping Methods...3 Copyright 2009 geographit. All rights reserved. The information contained in this document is the exclusive property of geographit. This work is protected under United States copyright law and other international copyright treaties and conventions. No part of this work may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording, or by any information storage or retrieval system, except as expressly permitted in writing by a corporate official of geographit.
Subdivision Mapping: Making the Pieces Fit Introduction Mapping new subdivisions is one of the key editing tasks required to keep tax parcels updated in a GIS environment. New subdivision lots need to be mapped within the boundaries of the parent parcel that is being subdivided. The methods used to map subdivision data determines whether subdivision lots will align with more recent high resolution digital ortho-photographic imagery and vector base map data acquired by photogrammetric methods after the subdivision was mapped. Even the slightest mapping and placement errors can cause the subdivision lots and derivative parcels to be misaligned by a noticeable margin (Figure 1). This distortion becomes apparent when the subdivision lot and parcel layers are viewed with recent ortho-photographic imagery. Parcel misalignment issues can lead to a lack of public confidence in the accuracy and quality of parcel mapping. So, tax mapping technicians frequently need to revisit the subdivided parcels in order to realign their boundaries to recent imagery. The effort can be considerable: in effect, this involves mapping the parcel boundary twice. This paper presents an alternative approach that ensures the subdivision mapping is of sufficient accuracy so that subdivision lot and parcel boundaries need only be edited once. Figure 1 Subdivision Misalignment Issue This paper presents a new method of updating parcel data using Global Positioning System (GPS) technology that was implemented in the Chester County, Pennsylvania. With this method, GPS is used to map X and Y coordinates of ground control points to accurately geo-reference the correct location of new subdivision lots so they do not need to be remapped later when new imagery or vector base map layers are acquired from photogrammetric sources. Subdivision Mapping: Making the Pieces Fit Page 1
Case Study: Chester County, Pennsylvania The use of GIS to maintain parcel data in Chester County began in the mid 1990 s with the undertaking of a digital tax map conversion project. Since its completion, all edits have been made to the parcel boundary layer within the GIS environment. As with any new undertaking, there was a learning curve to finding the best procedures to obtain satisfactory results. Satisfactory results with regard to subdivision mapping would be that the parcel boundaries align with other base map layers, especially recent digital ortho-photo imagery. From the mid 1990 s until 2001 the only available digital ortho-photo imagery was from a 1990 flyover, leaving no way to check the alignment for subdivisions added after 1990. The first opportunity to check the status of five plus years of subdivision edits occurred in 2001 when the year 2000 digital ortho-photo imagery was received. When the parcel data layer was viewed with the year 2000 aerial imagery that was received, it became apparent that the editing techniques for new subdivisions were not producing the results that were expected. A majority of the subdivisions that were mapped showed some sign of misalignment with the new ortho-photo imagery. These misalignments were noticeable enough that the public started to question the accuracy of the data when ordering custom map products. These misaligned subdivisions had to be remapped and realigned to the new photos. There was such a large amount of clean-up to handle that it took one year just to fix all the misalignment issues. New Approach to Subdivision Mapping Chester County needed a proactive solution to the subdivision alignment issue in order to maintain high quality mapping standards to avoid the added work whenever new aerial ortho-photo imagery is received. The clean-up of the previous mapping issues led to a better understanding of the mapping process and how to improve it. It also gave insight into the steps that introduced errors into the subdivision mapping process. Three primary sources of errors were pinpointed as contributing to inaccurate subdivision mapping results: Error Source 1: During the initial conversion to digital format from the source documents, Coordinate Geometry (COGO) is used to create the parcel lines by entering the metes and bounds (survey bearings and distances) that are annotated on the recorded subdivision plans and deeds. This is a time consuming process and there is always the chance of human errors when manually entering metes and bounds. Error Source 2: By retaining the boundary of the existing parent parcel that is being subdivided. The digital parcel boundaries were originally compiled to an accurate base map in the early 1990s, so it was thought that parcel boundaries were sufficiently accurate so that subdivisions could be mapped by fitting the new lots within the parent parcel s boundary. This resulted in subdivision lots being trimmed or extended to fit to these inaccurate parent parcel boundary lines. A decision was made to remap the parent parcel boundary by replacing it with the boundary rendered from the highly accurate COGO process, and editing surrounding parcels to fit to the new parent parcel boundary. Subdivision Mapping: Making the Pieces Fit Page 2
Error Source 3: Even though the COGO process renders a highly accurate representation of a subdivision, the subdivision lots need to be rotated and translated (geo-referenced) so they fit into the parcel fabric and base map. In the past, this was accomplished by either using the inaccurate parent parcel boundary corners as common points. Without identifiable features on the outdated digital ortho-photo imagery, it is uncertain whether the alignment is correct until newer imagery is acquired. The focus of the new approach was to address each of the three sources of errors with an emphasis on acquiring high accuracy control points to geo-reference the subdivision. Control points were clearly identifiable in the form of property corner markers shown on the recorded subdivision plans. It was a matter of obtaining high accuracy geographic coordinates of these markers so they could be used within the GIS environment for geo-referencing the subdivision plan. Global Positioning System (GPS) technology provided the means of getting the X and Y coordinate values of the markers. By doing a field visit to the site of the subdivision being mapped and locating the property markers the GPS is used to map the location of the control points. The mapped control points are then used to accurately and confidently position the subdivision within the parcel database. Table 1 Comparison of Old and New Subdivision Mapping Methods New Method Parent parcel polygon is remapped along with the subdivision lot lines Use GPS to obtain accurate geographic coordinates of subdivision boundary corners shown on the subdivision plan Surrounding parcel boundaries are edited to fit the accurately mapped parent parcel boundary to create seamless parcel boundaries with no gaps or slivers Old Method New subdivision lots are mapped and fit within the existing parent parcel boundary Existing base map data and inaccurate parcel corners used to geo-reference the subdivision plan Once positioned within the existing parent parcel, subdivision lot lines are extended or trimmed to create a seamless boundary layer Using GPS in the Subdivision Mapping Process In order to apply GPS technology to subdivision mapping, Chester County purchased a pole-mounted Trimble Pathfinder ProXT Receiver/Antenna and a Trimble Recon handheld field computer to run the Trimble TerraSync data collection/maintenance software (Figure 2). The hardware and software package chosen allows for quick and easy field data collection and easy to use procedures to download and postprocess the data once back in the office.. Subdivision Mapping: Making the Pieces Fit Page 3
Figure 2 Trimble GPS Equipment Selecting and Mapping Property Markers Selecting the control points to map with the GPS begins by reviewing the recorded subdivision plan. It is important to note that because Chester County is experiencing some of the highest numbers of recorded subdivision plans in Pennsylvania, there is a constant flow of recorded subdivision plans to be mapped. Because of the large number of edits, it was decided that GPS would only be used to map control points for major subdivisions consisting of 5 or more lots. The major subdivisions plans are reviewed to note the location of property markers that were placed by the surveyor at the time the property survey was completed. Most times, surveyors will mark the location of these property corners with wooden stakes and orange tape to aid in future recovery, so finding these property corners is fairly straightforward. The number of control points that will be mapped with GPS depends on how many are marked on the plan and the number that can be recovered in the field. A higher the number of corners that can be mapped results in a greater level of confidence in geo-referencing of subdivision plans. Ideally, it is possible to obtain a minimum of three control points located in opposite corners of a subdivision. However, even a few control points are better then none. Subdivision Mapping: Making the Pieces Fit Page 4
Figure 3 Candidate Property Markers on a Subdivision Plan Recovering property markers in the field can sometimes be a challenge. To aid in the search, GIS layers can be uploaded onto the Trimble Recon handheld device as a background map display in the field. In Chester County, it was useful to render the subdivision plan lines using COGO and then geo-reference the plan to its approximate location using the parent parcel and base map information. The subdivision plan was then loaded onto the handheld device and taken into the field as an aid in navigating to the approximate location of the property markers. Figure 4 Different Types of Property Markers Subdivision Mapping: Making the Pieces Fit Page 5
Fitting it all Together Once the property marker is recovered, its geographic coordinate (x, y) is obtained using the GPS equipment. TerraSync software can easily be configured with custom data input forms to support field data collection. So, Chester County configured a drop down selection list that made it easy to select the type of property marker (Figure 4) at the same time the GPS coordinate was being logged. To ensure the highest possible geographic accuracy of the GPS coordinates, the GPS data should be differentially corrected in order to remove errors contributed due to atmospheric conditions and the number of available GPS satellites used to calculate each GPS measurement. Differential correction involves processing each signal received at a base station of a known geodetic location to determine the difference between its GPS measurement and its known location, then applying the difference as a correction factor to the field measurement obtained by the roving GPS receiver for the same signal. This is a critical step to ensure the best quality GPS data. Using the GPS coordinates and the subdivision lot lines created by COGO, it is possible to geo-reference the subdivision lots to their true geographic location within the parcel boundary GIS layer. Geo-referencing is the process of defining links between the GPS points and its equivalent location shown on the converted subdivision plan (Figure 5). These links are used by the GIS software to apply mathematical rotation and translation factors so that the subdivision plans fits exactly to these GPS coordinates. When geo-referencing, it is important to select all the subdivision lines created with COGO including the parent parcel boundary and geo-reference them simultaneously; otherwise, topological errors could be introduced if different elements of the subdivision plan are geo-referenced independently of each other. Figure 5 Geo-referenced Subdivision Using GPS Control Points Subdivision Mapping: Making the Pieces Fit Page 6
Additional Benefits The parent parcel boundary converted from the subdivision plan is a more accurate rendering than the parcel boundaries of surrounding parcels that were originally converted from paper tax maps using best fit techniques. Therefore, to create a seamless parcel boundary layer, the surrounding parcel boundaries are edited to geometrically coincide with the parent parcel boundary from the subdivision plan. In 2006 Chester County received new aerial photos that were taken in the spring of 2005. This was the time when it would be determined if the new mapping process using the GPS was creating acceptable results. As expected, when the parcel layer was overlaid on the new photo, there were absolutely no misalignment issues with the subdivisions that were mapped using the GPS procedures. Major subdivisions frequently involve construction of new roads. By implementing the procedures discussed in this white paper, the road centerlines and associated rights-of-way can be converted using COGO at the same time as the subdivision lots. Since subdivision lots map to their correct geographic location as a result of the GPS control points, so will the new road centerlines and rights-of-way. This eliminates later alignment issues of the road layers as well (assuming road construction follows what was shown on the subdivision plan). Also, when in the field collecting property marker locations, it may be possible to update other GIS data layers such as manholes and fire hydrants, or to validate property addresses. By making the best use of time in the field, the costs of having staff in the field are minimized while at the same time the map accuracy and quality of data are increased. Time Savings and Public Perception The primary time savings of this approach is realized whenever new aerial orthophoto imagery is acquired. If the procedures described in this white paper are followed, it will be unnecessary to realign tax parcels, road centerlines, and road rights-of-way for major subdivisions. In turn, this will eliminate complaints from the public regarding data inaccuracies. The up front cost of doing it right the first time are repaid by later time savings not having to realign the data while also increasing the public s perception that the county has a high quality GIS program. Conclusion A few changes to the subdivision mapping process resulted in more accurate updating of the GIS parcels, road rights-of-way, and road centerline layers. By implementing the changes presented in this white paper, any information shown on recorded subdivision plans can be mapped with confidence, knowing that they will align with other base map data layers and future digital ortho-photo image updates. The key to the new process is that new subdivisions are no longer forced to fit into the existing parcel fabric; instead, they are placed were they truly belong based on geographically verified control points. Following these best practices eliminates later realignment efforts and vastly improves the quality of subdivision data. Subdivision Mapping: Making the Pieces Fit Page 7
Subject Matter Expertise geographit has considerable expertise in land records projects for county governments. The firm has designed and/or implemented GIS databases, tools, and maintenance procedures for a number of clients. Bucks County, Pennsylvania geodatabase design, data migration into a geodatabase, custom subdivision and parcel maintenance tools, Intranet webbased parcel viewer, documented data maintenance procedures, and training. Chester County, Pennsylvania geodatabase design, data migration into a geodatabase, Act 319 Clean and Green GIS software, Intranet web-based parcel viewer, documented subdivision and parcel maintenance procedures, and on-site staffing to assist parcel maintenance. Dauphin County, Pennsylvania geodatabase design, conversion of bearings and distance annotation from scanned tax maps, documented parcel maintenance procedures, and Act 319 Clean and Green GIS software Lackawanna County, Pennsylvania Quality Control/Assurance procedures during a tax map conversion project, Intranet web-based parcel viewer, and onsite staffing to assist parcel maintenance. Lebanon County, Pennsylvania geodatabase design, Quality Control/Assurance procedures for tax map conversion, data migration to a geodatabase, custom parcel maintenance tools, documented data maintenance procedures, and training. Lehigh County, Pennsylvania coverage database design, Quality Control/Assurance procedures for tax map conversion project, custom parcel maintenance tools, documented data maintenance procedures, training, and Act 319 Clean and Green GIS software. Northampton County, Pennsylvania geodatabase design, data migration, custom parcel maintenance tools, documented data maintenance procedures, and training. Subdivision Mapping: Making the Pieces Fit Page 8