Geomatic 101
The raster data
The raster data is often associated with images (digital photos, etc). It can be visualized with visualization softwares such as MS Paint, ACDSee and Photoshop, but these softwares can’t maintain the georeference (geographic coordinates). Examples of files in raster format are: .pdf, .tif and .jpg. Raster data in GeoTiff format require a Geographic Information System (GIS) software to maintain the georeference information.
This data is called raster because each pixel is expressed with a value representing a color or grayscale level. The raster data is thus represented in a grid matrix made of color values. Here is an example of raster data:
| 12 | 23 | 211 |
| 213 | 76 | 10 |
| 144 | 250 | 123 |
Figure 1.
As observed in Figure 1, the pixel values are represented under an 8 bits system thus a limit of 256 colors for each matrix. A numerical value (0-255) corresponding to a specific color is assigned to each grid cell. Moreover, as shown in Figure 2, the visual quality of the raster image decreases as we zoom in. Hence pixels become more visible as we zoom in.
Figure 2.
The vector data
The vector data is a digital representation made up of geometrical objects (points, lines and surfaces) having attributes of form, color and position. To display this type of data, specialized softwares of Geographic Information Systems (GIS) such as Arc Map, MapInfo, and AutoCad are necessary. Examples of files in vector format are: ESRI/.shp, MapExport/.E00, MapInfo/MID-MIF/.tab, AutoCad/.dxf, and Geography Markup Language/.gml.
As observed in Figure 3, each zoom in vector data will involve data processing. An advantage of vector data is its capacity to zoom in without loss of quality. Another advantage is the possibility to display the information by layer. It can be modified, for example, adding an attribute or modifying its graphic representation. It allows advanced spatial analysis on data, for example, measuring the length of a road segment or determining the area of a lake.
Figure 3.
Other information
Informative Web sites on image formats:
- http://en.wikipedia.org/wiki/Raster_graphics;
- http://www.ccdmd.qc.ca/ri/numerixel/principes-base/principes-base_1_1_1.html (in French only);
- http://en.wikipedia.org/wiki/Vector_graphics.
See also the GeoGratis Web site FAQ section to obtain a list of the most frequently asked questions. One will find a list of free “Open Source” GIS software which allow viewing these files while maintaining the georeference information.
Products resulting from the most popular digital topographic data on this Web site at the 1:50,000 scale are:
- CanVec (vector data in .shp or .gml formats);
- Toporama (raster data in GeoTiff format);
- CanImage (satellite images in GeoTiff format).
There are also various types of data varying from 1:10,000 scale up to 1:10,000,000 scale. To obtain the list of all products available on the GeoGratis Web site, click on “All Collections” in the left vertical menu.
