Transformations - Spatial Mathematics: Theory and Practice through Mapping - page 71

• Vector image: Limitations
• Often these images are dificult to align with raster photographs,
such as aerial images.
• Vector formats may be more dificult to edit than raster formats.
Extra filters may not be available.
• Vector images must be converted to raster images in order to dis-
play on a website.
For the purposes of mapping, there are two very important issues:
• If a photograph is to be combined with a map, bear in mind that
alignment of the photograph with the map will need to account for
the raster on which the photograph is based.
• If scalability is important, bear in mind that vector images are the
only truly scalable images.
No one format is “best.” Taking advantage of multiple options requires knowl-
edge and planning. Think about what is important as a final product before
embarking on mapping: If scalability is critical, as it often is, choose a vector
format. If combining a photo in with a map is important, consider a raster
format. The decision is yours. Again, there is no “perfect map” . . . not in paper
format, and not in digital format. The “Mapmaker's” quest for perfection can-
not be realized!
3.4.1 Representing the Earth using raster and vector data
Both vector and raster formats have long been used to represent data in a
GIS. Vector data are used to represent mostly discrete phenomena such as
points (such as water wells or volcanoes), lines (such as rivers or pipelines),
and polygons (such as lakes or census tracts). Once again, scale matters: At a
small scale, cities or rivers might be best represented as points, but at a larger
scale, as polygons. Raster data are used to represent continuously changing
phenomena, such as elevation or snowfall depth. However, rasters are also
used to represent thematic map layers, such as types of soil, zoning in an
urban area, or land use. And even more commonly, rasters are used to rep-
resent remotely sensed imagery, including aerial photographs and satellite
images. Rasters are used with imagery not only to represent the reflectivity in
the visible band of the electromagnetic spectrum, but also in bands that the
human eye cannot see, such as radar and infrared. These images are used to
detect ore bodies, to map urban infrastructure in 3D, to analyze the response
of forests to pine beetle infestation, and much more.
Resolution in both the vector and raster data are tied to the scale at which
that data set was collected. Simply because nowadays one can zoom in to a
fine resolution (large scale) on the screen where one is running GIS software
does not mean that the data in that GIS has been collected at that scale. For
example, one can zoom in to a scale of 1:1000 in a GIS, but the data may have