Digital Tax Parcel Mapping - Principles and General Procedures

	For the manual preparation of tax parcel maps a base map covering the whole or part of tax administration jurisdiction has often been developed from reference maps and reference materials. Base map preparation requires the combination of all existing survey documents, sketch plans and property information. The maps should be updated and drawn to scale showing political boundaries, property lines, public and private infrastructure such as roads, highways, railroad lines, irrigation network and natural features like rivers, lakes etc. It is important that all cadastral base maps are secured from the cadastral authorities in order to base the parcel maps on proper geographic coordinates. Manually drawn tax parcel maps are composed of an overview index map and individual tax map sheets of a standardized size showing blocks and sections of adjacent parcels. The index map shows location and boundaries of the individual tax map sheets in relation to major features such as roads and rivers. A digital tax parcel or cadastral mapping system requires the following components: A geodetic control network; An accurate base map layer (ideally, photogrammetrically derived) that is tied to the geodetic control; A tax parcel cadastral layer delineating all real property parcels; High resolution aerial photographs or high resolution satellite images to identify important property features; a unique parcel identifier assigned to each parcel; and Geographic layers of interest to the assessor such as administrative boundaries, zoning areas, rivers and flood plains. Information about the parcel, such as identifier, ownership, and assessment data are linked into the GIS from the tax administration system. Digital Tax Parcel Map Conversion A precondition for creating a digital tax parcel map is to define the desired spatial accuracy. Principally four different types can be distinguished, ranging from simple scanned maps to very accurate parcel maps based on the direct conversion of the primary field survey data (COGO or Coordinate Geometry based on the geodetic bearings, distances, and curve data established during the cadastral field survey). Choice and decision among these types of digital tax maps does not have to be a final one. For a digital tax parcel map project a town or tax jurisdiction can decide to proceed through all four types over a period of years depending on funds, time pressure and available cadastral documents. However, the fourth type, a COGO-based cadastral or tax parcel layer, is by far the most accurate. Scanned Maps The most simple and inexpensive form of a digital tax parcel map is to scan the existing manual maps into a raster image. This can be done with scanners or digital cameras. These maps are difficult to maintain (e.g. splitting of parcels). However, they can be easily accessed and shared with anyone with a computer, and are inexpensive and quick to produce. Scanned Maps with Data Points The scanned maps can be augmented with a data point placed in each parcel’s approximate centre. Still these maps are difficult to maintain, but the data points can be linked to data about the parcel from the database. Data can be displayed and queried. The scanned maps can be geo-referenced and used with other GIS data layers. This procedure may be the method of choice in cases with low spatial accuracy of the existing base data, time pressure to produce quick results for a large number of parcels and ill-defined property boundaries. Such a situation is often found in informal urban settlement areas or rural areas without mechanized farming systems. Digitized Parcel Polygons Manual maps (in paper or scanned form) can be traced to create digital polygons with associated parcel data. The output is a cadastral parcel layer that is adequate for most appraisal and planning analysis functions. Since parcel boundary lines do not have bearing and distance attributes, they usually are less accurate than parcel boundary lines created with the fourth method. Coordinate Geometry (COGO) Parcel Polygons Metes and bounds on cadastral field survey documents can be used to create parcels through coordinate geometry methods. Such maps are the most accurate and useful, the most easily maintained over time, and have a bearing, distance, and curve attributes for most boundary lines.
	A typical procedure of digital tax parcel map generation consists of the following steps: Step 1: Scanning or Photo Capturing of Existing Manual Parcel Maps The paper maps will be scanned or photographed with a good quality digital camera. Special care has to be exercised to avoid distortion. Step 2: Image Processing The next step is to assemble a raster image that combines all images or scans of the individual parcel maps into a single parcel mosaic which covers the whole tax administration jurisdiction or the area of the tax mapping project. Step 3: Image Georeferencing (Image Warping) Adjustments have to be made to enhance the “fit” or “overlay” of the composed parcel map with the topographic base map features. Often a GPS field survey will be done prior to the tax mapping project in order to update control points and major physical reference features such as roads, irrigation canals or power lines. An additional control and rectification option is to use high-resolution satellite images or aerial photos as a backdrop for the geographic adjustment of the digital parcel map. Up to date high-resolution satellite images or aerial photos allow also the digitization of building foot prints of the property. This is a very important additional graphic element of a digital parcel database, since buildings commonly are subject to real property taxation as well. Step 4: Digitizing of Parcels The parcel image mosaic is converted into a GIS usable parcel file by digitizing all parcel boundaries. This is done by “Heads-up” or “on-screen” digitizing.
	Step 5: Labelling of Parcels with a Unique Parcel ID Number to Create a Link to the Parcel Database The unique Parcel Identification Number (PIN) links the mapped parcel polygon with the tax database. Normally a geographic code is used as PIN in order to identify the administrative location of the property.
	Step 6: Creating Thematic Maps such as Landuse Map, Payment Status, Type of Ownership, Assessed Value, Market Value, Tax Delinquents and others A major advantage of digital tax mapping is the generation of both standardized as well as custom made thematic maps. Examples are theme maps showing land use, payment status, type of ownership, assessed values market values or type of property improvements depending on the variables stored in the database. For the tax officer it is most convenient when these maps can be accessed within the tax administration system. An important task during the build up phase of the digital tax database is the identification of non-plausible values and the checking of the digital database on encoding errors or faulty records of the assessor. Colour coded maps show extreme and possibly faulty data records immediately. Once the parcel database has been linked to the GIS, the following problems require attention: Parcel polygons with no matching database records, i.e. no Property Identification Number (PIN) related to polygon. Database records with no matching parcel maps. Wide variance between the GIS created parcel area and the surveyed area in the parcel database. Multiple claimants of the same parcel.