GIS File Formats

Geographic information systems reflect the phenomena that exist at a certain location on the earth's surface through many files or what are known as layers. Each layer corresponds to a particular type of geographical phenomenon. For instance, when representing a certain city's neighborhood, we sketch roadways in one layer, residential structures in another, trees in a third, etc. If we display all these layers simultaneously on the screen, we have a representation of the actual reality in this region.

Two main models are used to represent data: (1) (linear or Vector Data) and (2) (network or cellular; Raster Data). Digital elevation models (DEM) and irregular triangular networks (TIN) are two more (sub) models used to represent 3D data.

Vector Shapefile

Vector linear data model is the representation of all phenomena within a layer as a sequence of coordinates, similar to a paper map. A point is x, y coordinates for a specific location and has no space or dimension, whereas a line is a succession of points given for coordinates and has a dimension (length) but no space and a polygon is a phenomena that expands in a certain space and is enclosed by a line. Thus, the linear data model comprises three forms of representations for phenomena: Point, Arc Line, and Polygon. The method of representing the same phenomenon may vary depending on the drawing scale used and the layer's area boundaries. When drawing a layer for the features of a city, for instance, each neighborhood will be represented as a polygon, whereas the entire city will be displayed as a point when portraying the country as a whole.

The Vector Data model has multiple advantages, the most significant of which are:

• Accuracy in representing the locations of phenomena.
• The size of data representation does not necessitate a large amount of computer storage space, either in RAM or on a Hard Disk.
• Calculations such as length, area, and perimeter are simple to perform.
• The possibility to rectify information entered in a timely manner.

On the other hand, it has two major flaws: data entry needs a great deal of effort and time, and good experience and high accuracy are required for whoever enters the data. Nonetheless, the Vector Data model remains the most prevalent in digital maps, particularly in cadastral and engineering applications.

Raster Data

The Raster grid data model is based on the idea of placing a grid of squares on a map so that if one of the squares applies to a specific sort of phenomenon, this square will carry a number that is identical in value to all of its counterparts from the squares that applied to the same phenomenon. However, if one of the grid squares corresponds to the second phenomenon on the map, this square will be assigned a second number (different from the number of the first phenomenon). This concept is comparable to the principle of photography, in which an image is composed of a large number of microscopic squares, each of which is assigned a certain color to represent a particular phenomenon; hence, the colors of the image vary depending on the phenomena depicted on it. The spatial resolution or resolution of a data file is determined by the limits of a single square (or pixel) in the file. The smaller the size of the square, the higher the clarity and the capacity to represent phenomena. The Raster (grid) model is characterized by its ability to represent continuous phenomena and the speed of data entry into a geographic information system, while its most significant drawbacks are its need for a large storage capacity and its relatively simple accuracy in spatial representation, which is dependent on the size of the square or cell (pixel). Also, its capacity for spatial analysis is inferior to that of the linear model. The Raster model is utilized in aerial pictures, satellite imagery, and simple scanners in general.

Through vectorization and specialist tools such as the (Vector to Raster) program and the On-Screen Digitizing procedure, the Raster model can be turned into a linear model.

Digital Elevation Model DEM 

Digital Elevation Model, or DEM, is a digital file that provides elevation data for a given geographical area. To depict the terrain or topography of the land surface in the area, the digital elevation model can be in the form of a vector (a set of lines consisting of the three coordinates x, y, z of each point) or a raster.

It is possible to obtain a Digital Elevation Model through a variety of sources or input data sources, such as:

• Measurements of the land area using scales, Total Station, or GPS devices, followed by the creation of a digital elevation model for the study region using computer applications.
• Contour maps (after numbering them on the computer).
• From Aerial Photographs.
• From Remote-Sensing Images.
• From free global digital elevation models.

In recent years, the latter type has become the most popular and widely used type of digital elevation model for various reasons.

• Easy to get (from the internet).
• Possibility to obtain it for free.
• They are global models that cover all parts of the Earth's land surface.

There are several global digital elevation models available for free, for example:

• GLOBE Model
• ETOPO2 Model
• ASTER Model
• SRTM Model

Triangular Irregular Networks TIN

Irregular Triangular Networks (or TIN) is one of the three-dimensional data representation methods in GIS, however, its use is currently less common than that of raster files. The concept of creating TIN is based on determining the locations of points and the value of non-spatial data (required to create the three-dimensional surface) and then connecting them with lines that represent a triangle between which the height can be calculated at any point on it, and from which a group or network of irregular triangles (in area and volume) are produced to form a triangle network or TIN between them. TIN cannot be considered an independent form of data representation methods because it consists of the linear model in representation (points, lines, and polygons). However, the method of storing TIN data differs slightly from the method of representing normal linear data, in that it is a semi-linear representation for data based-vector.

From point, line, or polygon layers, TIN files can be developed. For instance, a point’s file including elevation values for each point in its non-spatial database (Attribute Table) can be turned into a TIN file that reflects the topography of the Earth's surface. Also, any form of non-spatial data can be utilized to represent the surface; for instance, a TIN can be generated to represent the temperature distribution or the depths of groundwater, etc. Also, a lines layer can be converted to a TIN file (if it has the values of the third dimension required to draw the three-dimensional surface, whether they are elevation or any other value).

The method of representing data using the TIN model needs far less storage space (on the computer's hard disk) than the raster method, and is therefore recommended for large-area surface representation.

Irregular Triangular Networks files are utilized in three-dimensional spatial analysis techniques, such as converting TIN to equal lines (or contour lines in the case of elevations), computing slopes and inclinations, and producing solids for the study area.

Raster files can be converted to TIN files and vice versa.