SIG represents the real world on the monitor as a sheet map with its map features (rivers, forests, gardens, roads, land parcels, etc.). The scale of the map determines the size and shape of the representation of its elements. The increasing scale of the map, the greater the size of its elements. SIG keep all its elements of descriptive information as attributes in the database. Then SIG to form and store it in relational tables.
Then SIG connect these elements with the corresponding table. So that these attributes can be accessed via the location map elements, and vice versa elements of the map can be accessed from its attributes. SIG connect a set of map elements with attributes within the layer units. Rivers, buildings, roads, administrative boundaries layer is an example. Consider the following illustration; Thus the design of the database is essential in the SIG. The design of the database will determine the effectiveness and efficiency of the input, management and output of GIS.
AutoCAD role in Geographic Information Systems (GIS) is more emphasized in the aspect of 2 (two) dimensions. Two-dimensional aspects referred to here is in terms of making and pendigitasian map as well as the setting up of the structure of the map expressed in layer. Things that need to be known in the operating environment of AutoCAD software for pendigitasian map are:
a) Elements in AutoCAD
Elements in AutoCAD is divided into two kinds, namely non-graphic elements and graphic elements. Graphic element is a series of commands to create an image or graphic form in the image of an object image, such as a line (l), polyline (pl), doublelin multiline (in / ml), retangle (rectange), circle (c), etc. , While the element of non graphic is a series of commands that serves to edit the images or objects in AutoCAD, such as the provision of number / letter (dtext), cutting line (trim), penembahan line (extend), deletion (erase), multiplication (array), etc. ,
b) Start the program AutoCAD
To start the AutoCAD program must first enter the Windows environment because pengopersian this program under the windows operating system. The steps that need to be considered are:
Making layer (image layer), intended to facilitate the imaging process and image editing.
Pengturan line type, line color, and line activation displayed.
c) Setting Environment Digitizing
The purpose of digitization disisni environment setup is setting the image function of each type of line in the map image. Commands the setup is done on the layer and which is intended to distinguish one line to another line.
d) Editing Process Map
As was mentioned earlier that the digitization process is a process of moving data manually (Map Manual) in the form of digital maps usually translated in the computer system. In this digitization process map used must really have a high accuracy (better than Map Bakosurtanal) both in terms of time and clarity of the information on the map itself.
The first step is to move the digitization of maps that will be digitized map coordinates into coordinates that can be read by a computer via the digitizer.
To be able to read the map coordinates that exist in order to be transferred to the computer system, then that should be done is to turn on the digitizer that can be read in the program AutoCAD. If the system has pengkoordinat can enter and read in the computer system, then the next is started digitization with regard arrangement layer of each type of layer that has been predetermined. For this pendigitasian orders are often operated is making the line a lot (polyline).
e) Storage of Digitizing
Storage of digitized can be done during the digitization process is in progress or when diiinginkan. The command used in the storage of digitized results are save or save as. Shape files saved in these results is the name of * .dwg. Furthermore, in order that the results of this digitization can be read and translated into the format Arc Info, then the * .dwg file format must be converted into the form of dxf files with ASCII type. After all the digitized map, then make the watersheds (pictured below.) As desired. Making the grid carried Auto CAD is useful as a method in predicting soil erosion conducted.
Figure 8. The process of making a watershed in Auto CAD