Digital Tax Parcel Mapping - Example: iTax Mapping in the Philippines
In the Philippines tax mapping is a more than twenty year old standardized procedure to assist the real property tax assessors in a comprehensive and homogenous field based valuation of all taxable land parcels in a municipality. It serves as a basis for a just and uniform taxation of real properties. Previously real property taxation was solely based on self declaration by property owners or claimants.
In February 2007, the GTZ Decentralization Programme in cooperation with the provincial government of Negros Oriental developed an Integrated Taxation Management System (iTAX) based on an intensive analysis of local needs and requirements, and focusing on real property tax.
Digital Conversion of Hard Copy Tax Parcel Maps
Two options were used by the iTAX project in generating the municipal digital tax parcel maps:
Step 1: Digital Conversion of Assessor’s Barangay Index Map, Section Index Map and Property Identification Maps (PIM)
As a low cost alternative to using a large scanner to reproduce digital copies of all paper maps at the assessor’s office, the use of a digital camera has proven to be an efficient and effective method. The recommendation is to use a good quality digital single lens reflex camera mounted on a tripod. It is very important, not only that an exact parallel camera angle is ensured but also that the paper maps are flat and smooth. The maps can be pasted on a wide wall, this helps to avoid camera tilt which in turn may cause image distortions.
Fig. 2 Example of a Property Identification Map
Step 2: Image Processing
Image mosaic/stitching of maps using Photoshop
Step 3: Image Geo-referencing (Image Warping)
The next major step in the process of creating the digital tax maps is the so called Geo-referencing (also called geo-coding). This is a process through which a set of data is assigned geographic coordinates, which in turn links them to known geographic locations on a topographic map. Geo-referencing involves defining a location in physical space by establishing a relation between raster or vector images to known map projections or coordinate systems. To geo-reference an image, one first needs to establish control points, input the known geographic coordinates of these control points, choose the co-ordinate system and other projection parameters and then minimize residuals. Residuals are the difference between the actual co-ordinates of the control points and the co-ordinates predicted by the geographic model created using the control points. They provide a method of determining the level of accuracy of the geo-referencing process.
Step 4: Digitizing of parcels/lots and attribution of Property Identification Numbers (PINs)
Second Method of Digital Conversion of Hard Copy Maps
This method is an alternative approach applied by the iTAX team to convert the hard copy parcel maps into a digital format. The method reduces the effort and time needed to create digital parcel maps because all the parcels are automatically drawn and there is then no need for geo-referencing and digitizing. An important advantage of this approach is that it creates the exact position, area and shape of the lot as surveyed in the field by the geodetic engineer.
Once the digital parcel map have been generated, the following steps are identical for both approaches mentioned above.
Step 5: Linking the iTAX database with the digital tax parcel map
A noteworthy novelty of the iTAX software is the direct integration of GIS maps into the application. In contrast many other similar attempts to establish a digital parcel mapping system have not proven sustainable because of the fact that the tax database is located in one system while the spatial database to produce the digital tax parcel maps is hosted in a separate GIS application. Experience has shown that tax officers are not inclined to handle two separate applications, particularly when one is also a demanding and complicated GIS software.
The implementation of GIS functions into the iTAX application is done by using the Developer Kernel (ActiveX version) of TatukGIS. This is done by object linking and embedding into the PowerBuilder development programme, which in turn is used for creating the iTAX application. Important to note is that the PowerBuilder (Sybase) software can read the map objects of GIS through TatukGIS (ActiveX).
Step 6: Creating/Lay-outing Thematic Maps
A major advantage of digital tax parcel 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, type of property improvements etc. depending on the variables stored in the data base.
These maps are integrated into the iTAX application which ensures that the maps always reflect the currently valid tax data.
An important application 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 highlight extreme and possibly faulty data records much easier than checking long tabular registers.