GIS By:fazilGeographical Information System – GIS What is GIS? GIS isan emerging method of data storage and interpretation.
GIS is,simply put a database. It is many tables of data organized byone common denominator, location. The data in a GIS systemis organized spatially, or by its physical location on the basemap. The information that is stored in the database is thelocation and attributes that exist in that base map, such asstreets, highways, water lines, sewers, manholes, properties, andbuildings, etc.
each of these items don?t just exist in thedatabase, the attributes associated with the item is also stored. A good example of this would be a specific sewer line, fromand arbitrary point A to a point B. Ideally, the sewer linewould be represented graphically, with a line connecting thetwo points or something of the like. When one retrieves theinformation for that line in particular, the attribute datawould be shown. This data would include the size of pipe, thepipe material, the upper invert elevation, the downstreaminvert elevation, the date installed, and any problem historyassociated with that line.
This is the very gist of what a GISsystem is. How is a GIS system created? Building a GIS systemfrom the ground up is a very time consuming and extremelyexpensive venture. This is why only large metropolitan areashave developed or are developing GIS systems. STEP1 -Determining and acquiring a base map Since a GIS system isbasically a digital map, the extents of the map have to bedetermined.
Once the area of interest is decided, the base maphas to be built. This is done using aerial photography (digitalorthography). This type of photography is very high definition,and of consistent scale (all photographs are taken from almostexactly the same altitude). The photography that is done isdifficult. Times when photographs are able to be taken are few. For instance, time is limited to the winter months due to lessfoliage, and from 10:00am to 2:00pm to reduce the shadows.
STEP 2 – Digital Overlay Once the base maps have beenacquired, the time consuming work begins. Each digital mapsection has to be gone over by a person on a computer. Thatperson outlines each object to be included in the gis system. These objects are the same as the ones stated previously (likesewer lines, water lines, etc.
), although they are not limited tothose. Along with all of the physical information that isentered into the database, there is a lot of other physicalinformation to be added that is not reflected on the aerials,such as property lines, tax and voting districts, as well as zoningdistricts. STEP 3 – Data entry Following the design andcreation of the basic data skeleton, the actual data need beentered. This includes all of the attribute data for all hydrants,sewer lines, water lines, properties, buildings, streets, highways,creeks, etc.
this portion of the process is the most timeconsuming. The bulk of the data to be entered is on paper, andthere is no easy way to convert it to digital other than manualdata entry. One issue in this step that is worth noting is theimportance of the quality control. As the data is entered,errors become inherent. In order to keep these at a minimum,an effective quality control system needs to be in place tomaintain data integrity.
STEP 4 – Application DevelopmentOnce all the information is entered (which never happenssince the aerial photographs become outdated in about onemonth), the core data is placed on a file server for the differentagencies to access. At this stage each agency usually directssome resources into application development. Since thestructure and organization is very generic, so all agencies canuse it, each entity develops applications for the data that isbetter suited for their purposes. For example, the metropolitansewer district would focus on the ability to query and store datarelating to the sewer system, since it is their main focus. Thecounty auditor would begin to restructure its file system andinformation management system to incorporate GIS into theirorganization, as well as developing tools within the GIS systemto streamline the commands to suit them.
STEP 5 -Continuous Improvements; Updates Since GIS is a databasethat is based upon land use, it is easy to realize how much inthe way of resources must be expended just to keep the datacurrent. Consider new construction. Every house and buildingthat has been
