|
2. What is the difference between raster maps and vectorial?
A raster map is the digital image that is received by scanning of an ordinary paper map.
As well as digital image it is the copy of original within the element (pixel) of scanning. If we will take
to account that a paper map is made with accuracy of charted objects not higher than 0,1 – 0,2mm, than scanning
with permission near 500 dpi (thus the size of pixel is approximately 0,1mm) provides saving on the raster map
all the details of initial paper map.
Raster map has two features. The first is that, as a rule, the file received after scanning has a large size.
Indeed, if one will scan the map in full-colored mode measuring 50х50 cm with permission of 508 dpi, the received
file will have a size 5000 х 5000 х 3 = 75Mb.
The second feature is that software is not able to do anything with a raster map, except for displaying.
Using a raster map computer can not accomplish such calculations as determination of optimum route, surface
contour, etc. Moreover, because of the large sizes, the displaying of raster maps on low-powered computers is
slow enough.
Enumerated features, that are the blemishes of raster maps, limit their usage substantially, but in restrained
terms their usage is vindicable because of low expenses and a high speed of production. If there are paper maps,
it’s easy and quickly enough to make electronic raster maps by scanning and following simple process of attachment
of raster image of map to one or another reference frames.Another blemish of raster maps is the impossibility of their
modification and making any changes.
Vectorial map on the internal content has nothing in common with a raster. It is a database that
has information about the objects of the map. This information consists of two kinds: geometrical and attributive
description of the objects. Attributive description includes information, for example, as the height of trees, the
width of the road, the speed of the flow of the river, the name of the settlement, etc. Geometrical description
determines the contours of the objects (in general they are curvilinear) as a rule, displaying them as broken lines,
which with an acceptable inaccuracy approximate the initial curves of the contour.
The main blemish of vectorial maps is substantial production outlay. For example, to create 1 sheet of vectorial topographical
map of scale 1:200 000, approximately it is required 2–4 working weeks, that, in its turn influences on the
final price of maps.
The undeniable advantage of vectorial maps is the possibility of automatic computer processing, which explains the
expansion of distribution in the various computer supplements, and also for navigation aim.
|
