introduction

Data input is the operation of encoding data for inclusion into a database. The creation of accurate databases is a very important part of GIS.

Data collection, and the maintenance of databases, remains the most expensive and time consuming aspect of setting up a major GIS facility. This typically costs 60-80% of the overall costs of a GIS project.

There are a number of issues which arise when developing a data base for a planning or management projects. The first issue is should the data be stored in vector or raster format. Considerations here include:
the nature of the source data e.g. it is already in raster form
the predominant use to which it will be put
the potential losses that may occur in transition
storage space (increasingly less important)
requirements for data sharing with other systems/software

As a general rule it is best to retain the maximum amount of information in the data base. If the data is available as points, lines or polygons then it should be kept that way. If a raster approximation of this data is also needed for analytical purposes then a raster version of the data may be kept in addition to the vector coverage. Many systems provide from quick conversion from vector to raster.

The issue of scale is often raised in relation to GIS data base development. It is important to remember that data stored in a GIS does not have a scale. Sometime people refer to a 1:25000 scale data base. What they mean is that the data has been taken from 1:25000 maps or that it has a level of accuracy which is roughly equivalent to that found on 1:25000 scale maps.

In line with the principle of keeping the most information possible the ideal is to fill the data base with data with accuracies equivalent to very large scale maps. This however may not always be practical as:
the data may not be available at very large scale,
it may be too expensive or time consuming to digitise from that scale ,
there may be no envisioned application that requires that accuracy;
so compromises are made.

Problems can arise when some of the data in a GIS is very accurate (drawn from large scale mapping – e.g. urban utilities) and other data is drawn from much smaller scale mapping (e.g. soils). In this case great care has to be taken that conclusions are not drawn on the basis of the less reliable data.

There are several methods used for entering spatial data into a GIS, including:
manual digitising and scanning of analogue maps
image data input and conversion to a GIS
direct data entry including global positioning systems (GPS)
transfer of data from existing digital sources
maps: scale, resolution, accuracy