There are four possible resultant image size:
thumbnail (150x150 pixels), small (200x200),
medium (400x400), and large (400x400). If you are
using a modem, you might want to use coarse image size to decrease the
image transfer rate. Remember, these are not "canned images" but GIS
datalayers that are translated to a mosaic-readable format upon
request, run time is not instantaneous.
The options include
It is very important to understand the role of the current Region
setting when interpretting the results of an Area Calculation.
This is for two reasons: 1) the larger the region setting, the longer the
process takes; and 2) the report analysis will be clipped to the selected
GRASS region. For example, if you were to do "Counties" against "Wetlands Categories"
with a region setting of "Marin County", your area totals would be restricted to the
selected Marin County region. The region setting is simply a pre-defined
bounding box around a named area and in the case of the "Marin County Region" it
would include all of Marin County and some of the surrounding counties as well.
The resultant Area Calculation Report would include Wetland Category totals for
Marin County and also for small sections of the surrounding counties that happen
to be in the Marin County Region bounding box. Thus your report could include a
value of "5 sq miles" for Napa County, when in fact the whole of Napa County has
not been included in the report calculations. Thus, it is very easy to misinterpret
these results if you don't understand the nature of the region settings!
The options include
There are several pages of forms for creating reclassification maps.
The first page selects the input base map to reclassify.
Subsequent pages select input categories to aggregate and the output
label for those categories.
Run time varies with the number of rings and the
resolution of the map.
Processes using coarser resolutions run faster and the resulting maps
take up less disk space. However, if you need to analyze these maps
at a finer resolution they will not support it. The metadata for a selected
raster map, if available, should indicate the data's resolution.
Display:
This creates an image from available maps in the database. It takes on
the order of 35 seconds to run. Once the image is created you can zoom in,
zoom out, pan across, spawn an external viewer, or query the base raster
map. The image is available for one hour, after which you will have to
recreate the image.
Area Calculations:
GRASSLinks can calculate the area of mapped information for selected raster maps.
You can find the total area of categories within a single map, or you can
find the areal extent of overlap of categories in two different maps.
If two maps are chosen, GRASSLinks calculates the total area for each
possible combination of categories in the first map and categories in
the second map. A report of areas in the Counties map, for instance,
would give a total area for each county. A report of Counties against
Wetlands to 16 Categories would show how much of each wetland type
occurs in each county.
Reclassification:
This GIS tool allows you to create a new map from an existing one
by combining categories from the chosen map into one new category.
For example, one could create a map of East Bay counties by aggregating
Contra Costa and Alameda from the Counties map.
Or a user could create a new At Risk map combining "low" "medium" and "high"
together into "at risk", keeping "urban" as "urban", and keeping "secure
greenbelt" as "secure greenbelt".
Combine:
This GIS tool allows you to create a new map that highlights the
overlap of information in two existing maps.
The output is a simple coincidence map showing one category where
the requested information in the two input maps coexists.
For example, one could create a map of tidal wetlands in Marin County
by choosing Marin from the Counties map and all tidal wetlands from the
Wetlands to 16 categories map. In that map, all areas outside of
Marin and all areas that are not tidal will be lumped together as a zero
category, while all Marin tidal wetlands will be one category together.
Buffer:
This GIS tool allows you to create a new map from an existing one
by building concentric rings around categories in a map.
The categories selected are aggregated together in the new map and
the concentric rings are built around those areas.
Raster Maps:
Raster maps are GIS data layers that are actually matrices of
information. They resemble images on a tv screen or photographs.
The map matrix has a resolution
associated with it and each pixel, or cell, has a data value.
Raster maps make nice base maps for displaying. They are also
very quickly read and manipulated by computer software because of
their matrix structure.
Vector Maps:
Vector maps use lines to denote features.
For one-dimensional features, such as stream centerlines, the vector
maps consists of lines. For two-dimensional features, such as lakes,
the vectors are area edges outlining the feature.
Vector maps draw nicely over raster base maps, but raster maps are
preferred by the GRASS software for complex data manipulation.
Site Maps:
Site maps are simply point locations. Features such as springs or pipe
outfalls can be described with site maps. They display as a dot or an
"x".
Region:
The region is simply the geographic area under examination.
For example, the whole Bay region or just one county. Region settings are usually
predefined bounding boxes that encompass a particular area, such as Alameda County.
A region can also be set to area of selected map(s). By selecting this setting,
a region bounding box that would be the minimum required to enclose the selected
raster map or maps (if more than one has been selected) is calculated on the fly.
Resolution:
Just as the information in a photo or on the screen looks smooth and
continuous, so does a raster map. However, if you look at a resolution
finer than the resolution that it was created, it will look blocky or
grainy, just as a photo does under a magnifying glass.
Since the REGIS geodatabase is for regional planning in the San
Francisco Bay and Delta area, most of the raster maps are at a
resolution of 50 or 100 meters. That means that at its finest, a
single raster grid cell represents 50 (or 100) meters on the ground.