is still relatively undisturbed. For this 
reason, great attention is devoted by the 
Government in conserving Palawan's natural 
resources. 
The purpose of the project is to delineate 
different areas in the watershed having 
various degrees of susceptibility to soil 
erosion. The GIS which was utilized by the 
project for this is CRIES, which is based 
on the Universal Soil Loss Equation (USLE). 
2.3 Assessment of Agricultural Resources 
Thru Photo-Tax Mapping 
Initiated in 1989 to assist local 
governments increase their revenue from 
agricultural lands, this project is being 
implemented in three (3) towns: Pagudpud in 
Ilocos Norte province; and Bulan and 
Sorsogon, both in Sorsogon province. These 
towns were chosen based on a number of 
criteria, foremost of which is the 
availability of new aerial photography. 
Property boundaries are delineated using 
rectified and enlarged aerial photos as 
base, and changes in declared ownership, 
land use and other improvements are noted. 
These data become the bases for revising 
and/or updating the tax assessments on the 
land. The resulting tax maps and tax 
declarations are then computerized using 
TYDAC SPANS GIS and Dbase 3+. 
Figure 1 shows the location of the study 
areas for all the three (3) projects 
mentioned above. 
3. METHODOLOGY 
3.1 Collection of Existing Data 
This is done in order to gather the layers 
of data required for the GIS. Information 
on topography, slope, soil type, land use, 
hydrology and climate/rainfall are commonly 
collected. Maps showing these are usually 
available from the appropriate government 
institution- 
Once this step is completed, the data gaps 
can be determined. Consequently, measures 
can be adopted to fill in these gaps, such 
as interpretation of available remote 
sensing data to extract the needed or 
missing information. 
For example, land use information can be 
interpreted from both aerial photos and 
satellite imageries. Distribution of soil 
types can be accurately mapped from aerial 
photos when supported by adequate field 
sampling. 
3.2 Preparation of Map Drafts 
Since a GIS requires both location and 
attribute, it is convenient and practical 
that all inputs should be in the form of 
maps. Fortunately, 1:50,000 scale 
topographic maps produced by NAMRIA are 
available for all parts of the country, and 
can be used as cartographic base. 
Drafts of the different maps are prepared 
initially, since these usually will have to 
be updated and/or validated in the field. 
782 
The medium used is tracing film, and 
blueprints are made for field mapping. 
Field visits are made for several reasons. 
One is to familiarize those involved with 
the project with the biophysical conditions 
of the area. Another is to collect data 
which may be required in the preparation of 
the various thematic maps. For example, it 
may be necessary to collect soil samples 
which will serve as "ground truth" data in 
the preparation of soil maps. 
For tax mapping purposes, extensive field 
work is undertaken to ascertain property 
boundaries. This requires visually 
inspecting and locating markers, also 
called monuments, and mapping these onto 
aerial photographs. 
3.4 Preparation of Final Thematic Maps 
The map drafts are revised or modified 
manually using field observations. Other 
required cartographic details are finalized 
such as legend, scale, coordinates, titles, 
etc. Colors are also added by hand. 
The final maps are produced in single copy 
only, since the manual cartographic process 
is tedious and painstaking. 
3.5 GIS Data Entry 
Data entry is accomplished both by encoding 
and digitizing map information, depending 
on the GIS used. Map digitization is 
commonly practiced for both ARC/INFO and 
TYDAC SPANS. On the other hand, data 
encoding is sufficient for the public 
domain software CRIES. 
It is worthwhile to note that the data 
generated by the three (3) systems are 
compatible in format. 
3.6 Data Analysis 
There are three (3) bases for data analysis 
corresponding to the three (3) application 
areas: Suitability Criteria, USLE, and the 
Real Property Tax Administration Code 
(RPTAC) of the Philippines. 
The Suitability Criteria is user-set, and 
guides the classification of an area into 
suitability types of high, medium and low. 
It takes into account slope, soil depth, 
soil texture, drainage, and soil fertility. 
Other considerations are existing land use, 
accessibility and hydrology. 
The USLE is pre-set into the CRIES 
software. Its variables are rainfall and 
runoff, inherent soil erodibility, slope 
length, slope steepness, land cover and 
soil conservation practices. 
The RPTAC provides rules for tax assessment 
of real property in the country. The tax 
due is a specific percentage of the 
assessed value, which is the product of the 
property’s area in hectares, market value 
and assessment level. The assessment level 
is a certain percentage which is dependent 
on the nature of the land use.