parcel mapping using GIS
Kalyan Gopal Shrestha[1]
1.0 Introduction
The concept of parcel mapping goes as far back as ancient
The utility of a parcel map is that it allows for easy identification of where a property is located relative to public ways and adjacent properties. An adequate parcel map should reflect size and shape of each individual parcel owned in a town.
1.1 Parcel mapping in
Although the present cadastral database is adequate for the day to day functioning of administration of land, the Survey Department targeted to initiate Numerical cadastre method to be used for mapping for greater accuracy, to prepare parcel based Cadastral Information System with a priority to regions of high population density for Land Information System and Preparation of Parcel Plan of land, and provide the data to the respective landowners.
Recently, a concept has been developed to prepare a parcel plan individually and also to carryout the survey using Total Station Theodolites in order to reduce the number of disputes, to increase the accuracy of mapping and to support the establishment of parcel based cadastral information system.
1.2 Digital parcel
Development of computer technologies along with geographic information systems (GIS) created opportunities to conduct this work more efficiently. It is much easier to store, maintain and update a digital map than an analog paper map. Digital parcel mapping also simplifies the process of locating the information about any of the parcels and markedly reduces time necessary for making changes and printing new copies of the maps on paper. Digital parcel maps can be integrated into a more general geographic information system that will allow users to maintain and retrieve the record of zoning, land use, conservation easements, etc.
2.0. Parcel Mapping Methods
One of the important steps in creating digital parcel maps is conversion of traditional paper maps (or analog maps) into digital GIS data layers. Not only do the data have to be transferred into a digital form, but they also have to be vectorized so that GIS software would be able to distinguish between individual elements such as lines, points, and polygons. There are several ways to do this:
- Manual digitizing from analog maps;
- Scanning with further “heads-up” digitizing or automatic vectorization;
- Coordinate Geometry conversion (COGO);
- Positioning property corners with GPS.
2.1. Manual Digitizing from Analog Maps.
This is the most basic method of digitizing traditional paper maps. Manual digitizing is done by placing a paper map on a digitizing board and entering all the elements of the map into the data base by means of a sensitive digitizing puck. An operator enters data by placing the digitizing puck over the points on the map attached to the digitizing board and pressing different buttons on the puck, thus indicating the type of each point. A point can be either an individual element, or a part of a larger element such as a line or a polygon. Lines and polygons are still defined by a set of points entered by an operator and connected by lines (vectors). Therefore, the accuracy of the data depends of the accuracy of the location of the points.
Digitizers are capable of providing a very high degree of accuracy in defining the location of each point (as high as 0.002” to 0.005”). Still, the accuracy of the data depends to a great extent on the accuracy and skills of the operator who manually enters the points by means of the digitizer. Usually, after all the data have been entered, it is necessary to edit them in order to get rid of operator’s mistakes.
Accuracy of the digital data also depends on the accuracy of the analog map. If lines on an analog map are misplaced, the operator will be able to correct them only to a certain degree. It means that most of the analog map inaccuracies will be very accurately reproduced on a digital map.
Manual digitizing has its own advantages and disadvantages as follows.
Advantages:
· The ability to correct errors or distortions in the original maps at the time of data capture.
· Highly reliable human recognition of map objects.
· The ability to interpret ambiguous or incomplete information and select the relevant required information at the time of data capture.
Disadvantages:
· The process is labor intensive and therefore very time-consuming and costly.
· The quality of results is highly dependent on the operator experience.
· The results may be inconsistent due to varying operator conditions, stress, and fatigue.
2.2. Scanning
The second method of parcel data conversion is scanning. In order to be scannable, a parcel map should be in a very good condition with minimum text on it. Hardcopy parcel maps are converted to digital form using a scanner.
Raster format is not very useful for many GIS applications. A raster image is simply a picture and can not be manipulated by the user. In order to be able to edit or update the map it is necessary to convert the raster image into a vector one. There are two methods that could be used for this purpose: automatic vectorization and heads-up digitizing.
2.2.1. Automatic Vectorization
This method could be used only if a map is in a very good condition – all lines are clear, have similar thickness, and clear intersections, and if there is no annotation or unnecessary features on the map. If this is the case, then the software can trace all the lines on the map and it would leave little editing to perform afterwards. This is the easiest and quickest method of parcel data conversion, but, unfortunately, parcel maps are usually not in perfect condition and have all types of different features and text on them. Vectorization software can not identify features (for example, it can not distinguish a water body from a parcel line), and most types of vectorization software can not distinguish text from lines. If a map cluttered by many different features is vectorized, it will require a lot of editing afterwards to delete all extra features and annotation. Time spent editing such a map could easily overweigh time spent on map conversion done by some other method. To minimize editing required after vectorization, it could be necessary to manually redraw a parcel map on Mylar selecting only parcel lines and thus excluding all extra features and annotation.
This method has certain advantages and disadvantages.
Advantages:
· Could be very fast and cost effective
· Relatively inexpensive
· Provides a very accurate representation of the analog map
· Easy
Disadvantages:
· False recognition of different features and text
· Editing could be very labor intensive
· The analog map needs to be in a pristine condition with minimum extra features and annotation
2.2.2. Heads-up Digitizing
This method is usually used when the analog map is in a very bad condition and when the digitizing board is not available. Heads-up digitizing involves manually tracing the lines on a computer screen over the top of the scanned raster image. The raster image is used as a background image. Everything that was said about on board digitizing could be applied to on screen digitizing as well. There are several advantages of this method
over on board digitizing:
1. Several people can do on-screen digitizing simultaneously because this work could be distributed between several computers, whereas it is very seldom that an organization would have several digitizing tables;
2. Heads-up digitizing is not as tiring as a board digitizing because an operator remains in a seated position, whereas on-board digitizing requires an operator to be in a standing position all the time.
2.3. Coordinate Geometry (COGO)
This is a more accurate method of parcel data conversion to a digital form than any of the methods mentioned above. COGO uses a completely different approach to the process of data conversion compared to digitizing or scanning. The difference is that the analog map is not involved, but land surveys and deeds are used instead. Land surveyors’ record information about each individual parcel in terms of geometric distances and angles from control points (benchmarks). The COGO procedure involves entering into computer such information as a precise starting coordinate, the direction and distances of each side of the parcel. After that, the computer performs some calculations to create a geometrically accurate parcel polygon. Unfortunately, not all parcels have property boundary descriptions or the information could be wrong. Therefore, it is often necessary to go out in the field and collect the missing information or verify the existing documentation.
COGO also has its own pluses and minuses.
Advantages:
· Provides an excellent positional and dimensional accuracy
· Provides a check on the closure (should start and end at the same point) and surveyed area of each parcel
· It is not necessary to have a hardcopy parcel map
Disadvantages:
· The most time consuming method
· Very labor intensive
· Very costly
· Deeds for all parcel must be accessible
2.4. Positioning Property Corners with GPS
The property corners of many surveyed and platted parcels are usually marked with iron pipes or nails. If coordinates of these markers are somehow obtained and input into GIS software, it would only be necessary to connect the dots in order to get a very accurate parcel map. The resulting accuracy could surpass even that of COGO (Struck 1998, np). Today, with the rapid development of the Global Positioning System (GPS)*, it has become relatively easy to get coordinates for the property corners. In order to obtain coordinates, two different strategies could be used: static positioning or kinematic positioning.
Advantages and disadvantages of this method are summarized in the table below.
Advantages:
· Provides the best accuracy
· Analog map is not required
· No guesswork
· All questions are resolved in the field, not in the office
Disadvantages:
· Obstacles can block or reflect satellite signals
· Very labor intensive
· Time consuming
· Requires training
2.6. Example of Analog and Digital Parcel Maps
Figure 2.6 illustrates scanned analog map. Apart from parcel lines, it also shows parcel numbers, acreage, parcels dimensions, street names, and other features. Figure 2.7 shows the same parcel map, transferred into digital form by means of digitizing. This map shows only parcel lines. All other parcels characteristics could be added to the attribute table associated with the map and optionally shown on the map.
Figure 2.7 Digital Parcel Map Figure 2.6 Analog Parcel Map
2.8 Selection of the Best Conversion Method
The choice of the particular method of parcel data conversion depends on the availability and quality of the source maps, availability of the specialized software, and the requirements for quality and completeness of the data vs. the cost of the project. Before initiating the process of conversion, one must evaluate the availability and quality of existing maps. Some of the questions that have to be asked are:
1. Were the maps originally drafted from deeds, surveys, and subdivision plats?
2. Do the original land base and parcel maps meet the GIS accuracy requirements
3. Have the maps been maintained on a regular basis?
4. Do the current maps visually edge match?
5. Are the current maps drafted at an acceptable scale?
If the answer to any of these questions is “no”, it might be necessary to recompile the paper maps before starting the process of conversion or use a method that does not require the analog map.
3.0. Georeferencing
A crucial element of any parcel mapping project is registering a parcel map with the correct real world coordinates. This procedure is called georeferencing. If the parcel maps are not georeferenced, no other information can be displayed over or positioned under the parcel map coverage. Georeferencing could be done either before or after the process of parcel data conversion. There are two groups of methods that could be used for this purpose depending on source materials available in each particular case:
1) Registration to a coordinate system; and
2) Registration to a base map
3.1. Registration to a Coordinate System
3.1.1. Use of Existing Coordinates
The first method of digital parcel map registration is to use the known coordinates. Currently, most of the towns in
Some of the towns may even have coordinates displayed in the corners of individual parcel maps. To use the known coordinates is the easiest way to register the map. If the known coordinates are used, maps become precise.
3.1.2. Use of Artificial Coordinate System
Unfortunately, not all towns possess maps registered to a known coordinate system. In such cases, artificial coordinate systems are frequently used. One way to create such a coordinate system is to assign the lowest corner a (0,0) coordinate and then calculate coordinates for all other corners of each parcel map on the basis of the distance from the original (0,0) point.
A parcel map created by this method will not be registered to any real world coordinate system but the distance between any parcels on the map will be accurate relative to each other. Afterwards it will be possible to register this map to some existing coordinate system or to a base map.
3.1.3. Use of GPS Technology
Another way to create the coordinate system for the town is to use Global Positioning System (GPS) technology. GPS receivers differ among each other depending on the accuracy they produce and their cost. It is possible to distinguish three types of GPS devices depending on their use:
1. The most basic “handheld GPS receiver”– Provides accuracy of several 30 m.
2. Professional, “mapping” grade receivers. Provide accuracy of several metre.
3. “Survey grade” or Phase based receivers. Provide accuracy of several metres.
There are several factors that can affect accuracy of the GPS readings:
- Equipment error.
- View of the sky may be obstructed by trees and other objects.
- Signals may bounce off the buildings, mountains, etc.
- The Department of Defense may turn on “selective availability”, which is an intentional clock error introduced into the timing signals.
One of the GPS applications is parcel mapping. GPS may be used to determine coordinates of easily identifiable positions on a parcel map (such as road intersection, property corner, etc.). In order to register a parcel map to real world coordinates it is necessary to have at least four coordinates. But in order to achieve greater accuracy usually it is better to have more than four coordinates. Another issue is the placement of the coordinates – if all four or more coordinates are concentrated in one corner of the map, it will be possible to register the map, but all other corners will be skewed and accuracy of the map will be very low. The choice of the number of coordinates in each case depends on the shape and complexity of each particular parcel map. In general, however, the more coordinates are used, the more accurate the final map will be. There are certain benefits of this method of parcel maps registration:
1. It is inexpensive
2. It is easy to use
3. It is not time consuming relative to other methods
4. It is not labor intensive
5. It is accurate
