Digital photo base maps, scanned paper
maps, and other raster maps are very effective base layers for internet map servers or
standalone desktop analysis systems. To overlay "vector" map layers on top
of "raster" maps, the raster maps must be georeferenced. Aerial or
satellite photo maps should also be orthorectified: ie, processed to remove distortions
due to terrain and camera optics.
Georeferencing means that the TIF, SID, JPG or other raster image file
must have a companion file containing the map projection (e.g., UTM, Polyconic, State
Plane) and the UTM, Latitude-longitude, or other real-world coordinates that correspond to
specific pixel coordinates in the image. The digital photo maps and other raster
maps supplied by SGSI are always shipped with georeferencing information in the form of a
MapInfo *.TAB, ESRI *.tfw, or equivalent file.
Top
three reasons to use ortho photo map layers from SGSI:
1. To create a better base map layer. Ortho-rectified
photo maps are the perfect "base layer" for the many layers of information that
comprise a modern GIS or desktop mapping database. The photographic details bring a sense
of reality to GIS maps and analyses. Because all SGSI-sold maps are supplied as
geo-referenced MapInfo map tables, all the normal street, census blocks, and other map
layers will overlay as you would expect. Nearly all users will appreciate the value
of a photographic base layer. Municipal planners, utilities, and public safety agencies
will especially benefit from having a "real" base map.
2. To validate or re-position streets and other map
information. Standard desktop mapping street maps, were built from 1:100,000 map scale
data such as the well-known U. S. Census' TIGER/Line files. These lines are
notoriously miss-positioned, but do contain valuable street name and address
information. Layer "1:12,000 scale" photo base maps from SGSI under
your street line maps. Errors will be easy to spot and easy to fix. SGSI-supplied
photo maps are based on photography taken between since 2000, so will show many streets
that do not appear in TIGER/Line files.
3. To create better site or vicinity maps. It
is one thing to show the demographic characteristics of the area around a site. With
SGSI-supplied photo maps you can visually gauge actual land use, the density of buildings,
and the residential or commercial character of a site. For example, with photo maps
you can "see" whether a park has trees, whether homes in the neighborhood have
swimming pools, and whether a waterfront area has a beach or an oil refinery. Real
estate consultants, environmental consultants, commercial site analysts,
telecommunications companies and others all need more information than normally shown on a
standard "vector" thematic map.
As you plan
your order, please consider the following issues:
1. System requirements:
Digital photo maps and raster maps in general are large
files. File sizes can be reduced if you specify compressed image formats such as
JPG, JPEG 2000, MrSID or ECW. Consider your computer processing power and disk
storage capacity. Do you have the horsepower to process image files
efficiently? Determine whether your software can display and correctly print maps
stored in compressed image formats.
2. Precision:
High precision orthos and high-resolution raster maps look better on the
screen but cost more and are substantially larger files. Know your application's
requirements, so you don't buy better images that you need.
3. Archival imagery:
Imagery is drawn from recent archives is usually cheaper and,
importantly, can be delivered faster than images that is collected on a special order
basis. Consider how important it is that your imagery be completely fresh. Archival
imagery is usually no more than 1 or 2 years old and is available for much of the USA.
Archival imagery for international areas is rare.
Rome, Italy
Not orthorectified
4. Areas of
interest:
Be prepared to specifically define your area of interest. Ideally, the
area of interest would be defined by a polygon or region map layer prepared in as a
MapInfo TAB or ESRI ShapeFile map layer. Photomaps are usually priced by the square
kilometer, however surcharges apply to highly-irregular project areas such as rivers or
pipeline/powerline corridors. The minimum practical order for digital orthophoto
imagery is usually 75 square kilometers or more.
5. Map projections:
Consider the map projection in which to request the raster
map. Raster maps, unlike more common vector highway or zip code maps, are not easy
to re-project. Some mapping software, e.g. ArcView 3.x, does not handle multiple map
projections very well. It may be important, therefore, to specify a map projection that
matches other map data you already have.
6. Cloud cover and shadows:
Satellite-based images often have some cloud cover and may
still be deemed acceptable. If you require absolutely cloud-free images, these may require
multiple collection attempts and costs will be higher. Satellite images are captured in
mid-morning, due to the satellite orbits and other technical factors. Consider the
effects of normal shadows that will appear in the image based on the season of year and
the latitude of your area of interest.
7. Your schedule:
The ortho process and fresh imagery collection may take up to 12
weeks, depending on weather and production backlogs. The fastest imagery to deliver
is archival imagery that is geo-referenced, but not ortho-rectified.
8. Non-returnable:
Because custom collection or processing is involved, photo base
maps are almost always sold on a non-returnable basis.
