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Methods of Digital Parcel Mapping

This is an excerpt from Digital Parcel Mapping Handbook. For more information go to the following web site:http://www.urisa.org/node/579

• Arnold W. Barnett, PE, LS – President, Mapping Technologies International, Inc
• Dave Boyd – Monmouth County GIS
• Brian Carson, GIS Specialist – Mercer-Somerset-Middlesex Regional Council
• Dr. Joshua Greenfeld – Surveying Program Coordinator-New Jersey Institute of Technology
• James Gresavage, Jr. – Department of Environmental Protection
• Bruce R Harrison – Hunterdon County
• David Nale -- ADR
• Richard Rehmann – GIS Division, Civil Solutions a Division of ARH
• John Thomas – Department of Environmental Protection
• Merrilee Torres – County of Burlington Data Processing, GIS Section
• Nancy von Meyer -- Fairview Industries

3.1 Introduction

As described in Chapter 1 of this publication the term Geographic Information System (GIS) encompasses the concepts of both automated mapping and database management, and uses computer graphics to show the spatial relationships. Chapter 1 also describes the roles that GIS plays as a vital tool for improved and expanded performance of job tasks and responsibilities of government, environmental, industrial, and utility users. While the potential of a GIS is overwhelming, the need to capture parcel information is fundamental to achieving that potential.

This chapter addresses the base mapping, parcel mapping and attribute relationships for digital parcel mapping. It begins with a discussion of control surveys, which are the spatial foundation of the base map and parcel mapping. This is followed by a discussion of aerial photography and the processes necessary to create a photogrammetric base map. Digital orthophotography can provide a base map to serve as a reference framework for the parcel information. Next a background discussion on Global Positioning Systems (GPS) technology is provided, since this technology is referenced frequently throughout this Chapter. Several alternatives for capturing parcel boundaries are then described: (1) Coordinate Geometry (COGO) and (2) conversion of paper maps. For the conversion of paper maps, several methods of registration of blocks of parcels to the selected coordinate system are described. This is followed in the last section with a discussion of linking attribute information to the parcel fabric.

3.2 Control Surveys

Control surveys are important for establishing a spatial reference framework for all parcel mapping. Regardless of the method of compilation, be it aerial photography, coordinate geometry or global positioning systems, the control surveys are a necessary first step. The density and accuracy of the control survey network may be varied and the way in which it is used in a parcel mapping project may vary, but it is essential and present in all GIS and parcel mapping projects.

A geodetic control network is the wire-frame or the skeleton on which continuous and consistent mapping and surveys are based. To understand the function of geodetic control it is important to realize that a map or a plane survey is a flat representation of the real, curved world. If the maps are to become an authentic representation of the real world we have to be able to "paste" small pieces of (flat) map contents onto a curved world. Geodetic control is the mechanism that enables us to perform this "pasting" accurately and consistently. Obviously, the need for geodetic control depends on the accuracy specifications of the map (GIS), the extent of area being mapped (the larger the area, the larger the deviation between a curved surface and a plane), and the desire for compatibility with other mapping or GIS projects.

3.2.1 Geodetic Datum

A datum is defined as any numerical or geometrical quantity or set of quantities which serve as a reference or base for other quantities. Traditionally, two types of datums are used: horizontal and vertical. 

A horizontal datum is a surface of constant values which forms the basis for the computations of horizontal control surveys. In a horizontal datum a reference ellipsoid is used as a mathematical approximation of the shape of the earth. Five parameters are required to define a horizontal datum: two to specify the dimensions of the ellipsoid, two to specify the location of an initial point (origin), and one to specify the orientation (i.e., north) of the coordinate system. The two main horizontal datums used in the U.S. are the North American Datum of 1927 (NAD27) and the North American Datum of 1983 (NAD83). In 1986, NAD83 replaced NAD27 because the latter was found to be not accurate enough to support modern positioning activities that occur in highly accurate electronic measurement systems and satellite-based positioning systems. NAD83 is an earth-centered datum and relies on an ellipsoid (and other constants) of the Geodetic Reference System of 1980 (GRS 80). It is important to note that GPS position calculations are based on the WGS 84 datum (World Geodetic System of 1984), which for all practical purposes is identical to GRS 80.

A vertical datum is a geoid that represents the best approximate mean sea level. Heights referred to the geoid are called orthometric heights, which stand in contrast to ellipsoidal heights, which refer to the ellipsoid. In the U.S. there are two vertical datums: the National Geodetic Vertical Datum of 1929 (NGVD29) and the North American Vertical Datum of 1988 (NAVD88). Superceding the NGVD29, the NAVD88 is a newly-defined and computed vertical datum, and provides a consistent, very accurate set of height values for mappers, surveyors, and geodesists. One should note that the elevation of a given point can vary significantly depending on whether it is expressed in NGVD29 or NAVD88 values.

Elevations are not required for most parcel mapping applications. However, since GPS is a 3D (actually 4D) measuring device, elevations are available for every point. This elevation data should be stored in the GIS. As mentioned earlier, the GPS-derived elevation refers to the ellipsoid (ellipsoidal height), not the mean sea level (orthometric height). In New Jersey the elevation difference between the ellipsoid (GRS80) and the mean sea level (Geoid) is about 100 feet. Therefore, before one uses any elevation data it is imperative to identify the height system on which the elevation is based.

3.2.2 High Accuracy Reference Networks (HARN)

The original NAD 83 geodetic network was computed mostly by using traditional surveying observations and methods. Very few GPS observations were included in the adjustment computation. The design and implementation of this network preceded the developments of the GPS technology and therefore the practical usage of these control points for GPS applications can be problematic. The first difficulty in using most of these control points is that they are not "GPSable." In other words, the points are located near objects that obstruct the required clear visibility between the receiver and the satellites. The second difficulty is that many of these points are located on mountain tops and other locations that are not easily accessible. To work efficiently with GPS one needs to have quick and easy access to control points. The third difficulty in using the original NAD 83 network is that control points are spaced irregularly. Hence, there is a high chance that there will be insufficient control points in the vicinity of your project. The final difficulty is that the original NAD 83 network is not accurate enough to serve as control for GPS observations. The most accurate horizontal standard in the original NAD 83 network is 1:100,000, as compared to a 1:100,000,000 accuracy attainable by GPS.

To eliminate or significantly reduce these problems, several states (including New Jersey) are developing, in conjunction with the National Geodetic Survey (NGS) a High Accuracy Reference Network (HARN). The HARN was designed to establish "GPSable" geodetic control points accessible 24 hours a day by car or light truck within, at most, 30 to 45 minutes travel from any point in the state. Once the HARN was established, a new adjustment was computed and the points in the network were assigned new coordinates different from those of the original NAD83 adjustment. The results of the new adjustment are named NAD83 (199x), where (199x) is the year in which the adjustment was completed (e.g., NAD83 (1998) was completed in 1998, etc.). Changes in positional (horizontal) coordinates from the original NAD 83 are expected to range between 1-3.5 feet; thus, the code-based GPS data collection will not be affected by the new values.

3.2.3 The State Plane Coordinate System (SPCS)

It is impossible to map a curved Earth on a flat map using plane coordinates (x,y or northing, easting) without distorting angles, distances, or areas. It is possible to design a map projection such that some of the three are undisturbed or minimally distorted. The State Plane Coordinate System (SPCS) is a map projection system that minimizes angular distortions if only a small portion of the earth is flattened out. Thus, the SPCS is a rectangular (x,y or northing, easting) coordinate system describing geodetic positions of a limited area (a state or a portion of it) on a plane. The (x,y) coordinates are computed by projecting latitudes and longitudes from a mathematical approximation of the earth (i.e., NAD83) onto a rectangular grid. SPCS consists of a set of mathematical relationships that are used to convert northing and eastings into latitudes and longitudes and vice versa. It also includes a set of formulas to compute the size and the direction of location displacement (positional error) resulting from the projection process.

By law, the New Jersey State Plane Coordinate System (based on NAD83) is the official survey base for the State of New Jersey (R.S.51:3-7).

3.2.4 Accuracy Standards for Geodetic Control

According to the FGDC’s latest National Standard for Spatial Data Accuracy (introduced in Chapter 2), the classification standard for geodetic networks is based on statistical accuracy instead of a ratio (such as 1:100,000). The accuracy Standards for Horizontal, Ellipsoid Height, and Orthometric Height are:

When control points in a survey are classified, they have to be verified as being consistent with all other points in the network, not merely with those within that particular survey. In other words, the accuracy standard of the geodetic network does not measure a local consistency of the survey points but it measures the ability of that survey to duplicate already established control values. The procedure leading to classifying a geodetic control network accuracy involves two general steps:

The survey measurements, field records, sketches, and other documentation are examined to verify compliance with the specifications for the intended accuracy of the survey. This examination may lead to a modification of the intended accuracy.

The computation of the geodetic network is performed with established Least Squares methodology. The results of the computations are statistically evaluated to determine the achieved accuracy and verify compliance with the sought accuracy. This analysis may lead to redefinition of the actual accuracy standard class or to repeat/add field measurements.

According to the FGDC, by supporting both local accuracy and network accuracy, the diverse requirements of National Spatial Reference System (NSRS) users can be met. Local accuracy is best adapted to check relations between nearby control points. For example, a surveyor checking closure between two NSRS points is mostly interested in a local accuracy measure. On the other hand, someone constructing a Geographic Information System will need some type of positional tolerance associated with a set of coordinates. Network accuracy measures how well coordinates approach an ideal, error-free datum. Thus, for control points both local accuracy and network accuracy should be reported in a metadata record.

3.2.5 Establishing a Geodetic Control Network

Establishing geodetic control to be used as a geodetic base for parcel mapping and GIS is not an easy task. It can be done with traditional surveying techniques but currently is done almost exclusively with GPS. Section 3.5 describes GPS Technology in more detail. The following steps are applicable to any technology. In general, establishing a Geodetic Control network involves six steps:

Determination of accuracy and density criteria.

1. Reconnaissance.
2. Monumentation.
3. Selection of method and means.
4. Field observations.
5. Computations and adjustments.
6. Documentation (possibly formatting for inclusion of the points in the National Geodetic Reference System).

The following describes each of the steps in the process.

Determination of Accuracy and Density Criteria

The most critical aspect in developing a parcel mapping project is establishing accuracy standards for the project. The accuracy standards must define the accuracy with which one can expect to locate parcels and related features in the GIS. In other words, if a property owner, a consultant or the local government employee point at a parcel corner as it appears in the GIS, he or she has to know if that location is accurate to within 100, 10 or 0.01 meters. This is a critical decision because it will determine the range of applications for which the GIS will be valuable and the cost of the parcel mapping project. Generally, more accurate parcel data can be used for more local government activities but also cost more.

Once the intended GIS accuracy is established, the accuracy of the geodetic control has to be set. While the accuracy of the geodetic control should be related to the GIS accuracy, it is a good idea to find out what it would take to have a more accurate geodetic control network. A highly accurate geodetic network will serve not only the current parcel mapping conversion project but also enable future enhancements and improvements to the GIS. An accurate geodetic network is also important if, in the future, any mandate will require mapping projects to be tied to the SPCS. At a minimum, the accuracy of the geodetic control should be three times higher than the parcel mapping accuracy standard. For example, if the declared parcel mapping accuracy is ± 1 meter, then the control network should have an accuracy of no worse than ± 0.3 meters. The factor of three provides a 99% statistical confidence that the location of the control is better than the location of the parcel data.

Another design parameter of the geodetic control is the spacing between control points. Here again there are short term (project specific) and long term considerations. If the intent in establishing the geodetic control is only to support the parcel mapping conversion, then the control points should be placed at strategic locations such as the perimeters of the georeferenced tax maps. If the objective of the geodetic control is to support a systematic ground-up parcel mapping, then control points should be scattered densely throughout the jurisdiction of the local government. In the latter, the location of the control points should make it easy to tie-in all new surveys or other mapping projects into a consistent and contiguous data layer.

Control points can be established in two phases. In the first phase a spare but very accurate and stable set of control points are monumented, surveyed, and computed. This network can be used to establish the wire-frame positioning skeleton for the GIS and for georeferencing the compiled parcel data. At a subsequent phase this network can be densified as an on-going process based on needs and availability of funds.

Reconnaissance

Site selection is a key element in reducing the time required for planning GPS surveys, processing, and analyzing the observed data. Sites that are not selected carefully could cause difficulties during initial survey and also during subsequent usage of these points in future surveys. These difficulties usually translate into unnecessary costs and delays. Some guidelines for optimal location of the control points are:

1. An all-around clear view of the horizon above 15° at normal antenna height.
2. Locate on stable ground.
3. Locate in a place that is readily accessible but not likely to be disturbed.
4. Anticipate future construction and tree growth.
5. Avoid tall artificial and natural reflective structures or surfaces that could cause multi-path effect on the GPS signal.
6. Accommodate for future use with conventional surveying equipment.

Monumentation

The physical evidence of a geodetic control network is provided by geodetic stations. Such stations are typically well-defined points in manmade monuments, landmarks, or natural features. A typical man-made geodetic control monument is established by inserting a 5-foot long, 10-inch diameter pipe into the ground, filling it with concrete and placing a metal disk on top. The disk is engraved with information about the agency or company that established the monument, an ID number and whether it is used for horizontal positioning, elevation, or both.

Selection of Method and Means

In today's technology GPS is the most appropriate method for determining the accurate positions of the geodetic control network. Only if the area is not GPS-able, such as occurs in downtown areas with densely built high rise buildings or in densely forested areas with heavy canopy cover, should conventional surveying techniques be considered. Non-"geodetic-grade" network (positional accuracies of larger than a few centimeters) points could be observed with high-end mapping grade GPS receivers. This is not recommended for parcel mapping since it is not necessarily appreciably less expensive and less time consuming than using established GPS surveying equipment and procedures. Therefore, surveying grade, preferably dual-frequency, receivers should be employed.

Field Observations for GPS Control Surveys

Field observation should follow a predetermined set of specifications. The particulars of the specifications depend on the sought accuracy of the control network. More accurate (geodetic level) networks will have more stringent specifications and vice versa. Some of the parameters that should be considered are:

1. The geometry of the satellites during the observation session (largest acceptable PDOP and the number of quadrants in which the GPS signal is available. PDOP is a statistical measure of the receiver-satellite(s) geometry. A small PDOP value is desired).
2. The minimum number of satellites that have to be observed simultaneously.
3. The acceptable degree of signal obstructions that exist around the control points (in terms of elevation angle above the horizon and azimuth).
4. The minimum/maximum station spacing.
5. The observation technique (static, rapid static, etc.).
6. The number of ties to HARNs or equivalent existing high-accuracy geodetic control.
7. The observation scheme (the order of session and the arrangement for receiver locations in each session and scheduling).
8. The number of times baselines have to be repeated or points re-observed.
9. The length of time for each observation session.
10. 10. The data sampling rate.
11. The antenna setup procedures.
12. The station description, including instructions for how to find the station.
13. Field or session data log requirements (recording session information including special events that occurred during observation, meteorological data, etc.).

Computations and Adjustments for GPS Surveys

Once the field observations are completed, the observation sessions have to be computed and analyzed. The computation of the observations is usually performed with the software provided by the receiver's vendor. When geodetic GPS observations are performed, baseline vectors (or distance and azimuth between receivers), rather than point coordinates, are computed. Hence, it is necessary to analyze the quality of individual baselines as well as the closures of baseline loops. A baseline loop is a set of sequential vectors that form a closed polygon that ideally start and end at exactly the same point. The loop closure quantifies the computed difference between the starting and ending points or the misclosure of polygon. Standards for computation should address matters such as:

1. Are precise ephemeredes required for baseline computation?
2. Is a "fixed" solution for the baselines mandatory or is a "float" solution acceptable?
3. The acceptable difference between repeated and unadjusted computed baselines.
4. The acceptable difference between known and observed baselines.
5. The number of loop closures.
6. The acceptable loop closure.
7. The minimum and maximum allowed loop length.

When the baselines computations are completed the entire geodetic control network has to be processed with a "least squares" adjustment program. The adjustment process will mold all the individual baselines into a single and consistent geodetic network. As before, the statistics produced by the adjustment program have to be carefully studied. Furthermore, standards have to specify acceptable criteria for the results of the network adjustment.

Documentation

 The documentation of the geodetic network is an important cornerstone of the parcel mapping metadata. The FGDC has established framework and metadata specifications for geodetic control, which we encourage everyone to follow. Some of the specifics of geodetic control documentation include:

1. Point description and location sketches
2. Observation data
3. Table of baseline computations with their statistics
4. Baseline computation analysis
5. Table of loop closures with their statistics
6. Loop closure analysis
7. Input/output of the network adjustment
8. Network adjustment analysis
9. Final adjusted coordinate listing with their statistics (accuracy, covariances, etc.)

After the geodetic control network is completed, the adjusted coordinates can be formatted and submitted to the National Geodetic Survey (NGS) for inclusion in the National Geodetic Reference System (NGRS). Submitting the results to NGS is termed "blue booking." This step involves some extra expense but has some benefits. The main benefit of blue-booking the project is that NGS will publish and maintain the network’s coordinates.

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