There are advantages and disadvantages to using a raster or
vector data model to represent reality.
Raster datasets record a value for all points in the area
covered which may require more storage space than representing data in a vector
format that can store data only where needed. Raster data also allows easy
implementation of overlay operations, which are more difficult with vector
data.
Vector data can be displayed as vector graphics used on
traditional maps, whereas raster data will appear as an image that, depending
on the resolution of the raster file, may have a blocky appearance for object
boundaries.
Vector data can be easier to register, scale, and re-project.
This can simplify combining vector layers from different sources. Vector data
is more compatible with relational database environments. They can be part of a
relational table as a normal column and processed using a multitude of
operators.
The file size for vector data is usually much smaller for
storage and sharing than raster data. Image of raster data can be 10 to 100
times larger than vector data depending on the resolution. Another advantage of
vector data is that it is easy to update and maintain.
In a typical GIS project, there is need to identify the
objectives of the project, create a database specifically for the project and
containing all information for the resolution of the problem, then, we need to
use GIS functions to create an analytical model that will be able to solve the
problem and lastly present the results of the analysis. ESRI (1996), define
four basic steps in the development of a GIS project.
Step 1: Identification of objectives
The first step of the procedure consists in identifying the
objective of the analysis. The following questions need to be taken into
consideration in order to define the objectives:
· What is the problem to be solved? How is the problem
being solved actually? Are there any more solutions with the aid of GIS?
· What are the expected results: reports, study maps,
presentation maps?
· To whom are the results destined: management,
technicians?
· Are the data going to be used for other goals? What are
the conditions in this case?
This is an important step as the answers to the latter questions
define the objectives of the project as well as the methods to be used in
putting in place the system.
Step 2: Creation of a database
The second step consists of creating a database for the
project. The creation of this database is a three stage procedure. The stages
help in the conception of the database, to automate and to assemble the data
and then manage this data. The design of a database comprises:
· The identification of the necessary spatial data with
respect to the need in analysis, definition of the different attributes
required by the various entities, definition of the boundaries of the study
area and the choice of the coordinate system to be used,
· Automating access to the data which comprises the
digitization or the conversion of data from other systems and formats to a
useable format, as well as the verification of the data and correction for
errors, and
· Management of the data which comprises the verification
of the coordinate system and union of two adjacent layers.
Creation of a database for the project is of prime importance
and usually takes a greater part of the time required for a GIS project. The
completeness and precision of the data used in analyses will equally determine
the precision of the results. This explains why this study is centered towards
the creation of a database for the irrigation system of the PHP group.
Step 3: Data analysis
This third step consists of analyzing the data obtained.
Analysis of GIS moves from simple realization of maps to the creation of
complex spatial models. A model is the representation of reality used to
simulate a given process, predict a given result or analyze a given problem. A
spatial model applies one or more of these three categories of functions of
GIS:
· Geometric modeling - calculation of distances, areas and
perimeters, creation of buffer zones,
· Coincidence modeling - superposition of various data to
find their locations or the coinciding values,
· Contiguity modeling- allocation, search for routes and
cutting into sectors.
With the use of GIS it is possible to carry out rapid
analysis which would have been extremely long or even impossible to do
manually. Different scenarios are being created by changing the method or a
parameter of an event and executing the analysis to obtain various results.
Step 4: Presentation of Results
This is the final stage of a GIS project. In most cases, GIS
results are better shown on maps. Diagrams and data reports are equally other
methods of presenting results. These diagrams and reports could be printed
separately, incorporated into created files or placed on maps.
