Introduction to Rasters
In ArcGIS 9.2
What can you do with Rasters
• Lots….
Impressive???
•
Two
Basic
Kinds
of
GISs
Vector GIS
– Objects represented
by:
•
•
•
•
• Raster GIS
– AREA represented
by:
points
lines
polygons
large database each
object
• Grid cells
• one value per cell
• Large number
thematic layers
Forest
road
Cropland
stream
5
324
The
Raster
Image
Problems with shape and resolution of
features
Is this
stream or
field?
The road is
straight but it
does not look
that way in the
raster version
Is this
sream,
road or
forest or
all three?
So we have to use layers!
Layers
Streams
Road
For streams and Roads sometimes
the non-feature can have a value
but is usually “no-data”.
Cover
Field &
Forest
What is a raster?
• A regular arrangement of cells (speadsheet)
whose values represent what is on the surface
• The cells can have two values
– Surface representation – 1 # per cell
– Location (computed from raster extent on the fly)
• This makes the raster a Geographic raster
• Not all are geographic
Making a Raster
The basic process is to lay a grid over the map so that you
can “rasterize” some or all the features. Here is the grid …
•Lets suppose
that we need to
make a raster or
GRID, to use AV
terminology, of
Lakes.
• Each grid gets
a value of what
is under it.
•There can only
be one theme
per grid layer.
Make a raster of lake
•Let’s color the lake
cells blue
•BUT which cells?
•The cells that are all
lake are no problem
•We need a rule
•Let’s say the cell has
to be at least 50%
lake to be a lake cell
•This is going to
create some spatial
error but that’s life in
the raster world!
•GO…
Make a raster of I90
•OK
•Now let’s do I90
•In this case the
road is much smaller
than the grid so we
can’t use the 50%
rule
•So we just say that
for a cell to be I90
it has to have a
“significant” piece of
the road in it.
•So…
Make a raster of I90
•Since this is a
thematic layer we
could add other
roads to the layer
•Or they could be
put on separate
grids
•Or you could
extract the needed
road data into a new
grid layer
•Here is Rt 20 added
Stack of rasters
Skewer of
Location
Streams
pipelines
Landuse
Roads
Raster cell size
• In the examples the size of the cells
were pretty large
• Especially for the roads!
• We can, of course, use smaller cells…
• But …
• Later
Rasters & data types
• Rasters
–
–
–
–
Soils (Nominal data) (Qualitative)
Temperature (Interval data) (
Elevation (Ratio data) (Quantitative)
Highways (Ordinal) (?)
• Since in ArcGIS 9.x it is easy to go
back and forth between raster and
vector representations so we can
make the best use of both worlds
Kinds of rasters
• Grids – spatial data (elevation etc.)
• .Tif – usually called GeoTiff if has
geogaphic properties (projection)
• .sid - MrSid – a raster that is
compressed (MRSID) Imagery
• .IMG - (ERDAS) imagery
• .jpg - imagery
Cell Size
• A fundamental question
– The smaller the cells (resolution) the
better the raster represents the ground
– But you pay a price for using small cells…
• Halving the resolution of a cell  quadrupling
of the number of cells
• And thus greatly increasing
processing times
Addressing raster location - cells
0
1
2
3
4
5
6
1
N-S
• Counting
cells
• Basic
addressi
ng has
origin at
upper
left
2
3
4
4,6
E-W
A raster can be up to 4 x 4 million grid cells
• X,Y
coordinates
• Origin at
lower left
Increasing Y (Northing)
Addressing raster location – real world
Increasing X (Easting)
In ArcGIS
• In ArcGIS most rasters are GRIDS
• There are two (2) basic types of Grids
– Integer
-Discrete data,
• Data range -3.438 to 3.431-1
– Floating point
-Continuous data
• Data range -2,147,483,648 to 2,147,483,647
• Grids can have the value “No Data”
Attribute Data
• Integer data will have an Attribute
Table
• Floating point data will NOT have an
Attribute Table
• You can
– Go from Real to Integer
• Int – Truncates
• Round up and Round down
– Go from Integer to Real
• Float
Extent of Grids
Layer 2
Layer 1
Layer 3
The Stack
•The boundaries of the
input layers can overlap
exactly, partially, or not
at all, but only the area
where layers overlap
comprises the stack.
•The stack’s BND is where
the boundaries of its
layers intersect.
•Some processes only
work on the stack data.
Properties
DEM
• Digital Elevation Model (sometimes called a
Digital Terrain Model (DTM)
• A basic grid layer for many applications of
GIS
• It is the GRID equivalent of a TIN
• As with all GRIDs it is a rectangular array
of cells.
• We will be using the DEM for the town of
Martinsburg in Lewis County, NY
Dem_lewisMart
2,155 columns by 1,408 rows
=3,034,240 cells
Added Rivers & outline
Hillshade grid makes Terrain clearer
Adding Contours