Introduction to Rasters

In ArcGIS 9.2

What can you do with Rasters

• Lots….

Impressive???

324

5

Two Basic Kinds of GISs• Vector GIS

– Objects represented by:

• points• lines• polygons• large database each

object

• Raster GIS– AREA represented

by:• Grid cells• one value per cell• Large number

thematic layers

Forest road

Cropland stream

The Raster Image

The road is straight but it does not look

that way in the raster

version

Is this stream or field?

Is this sream, road or forest or all three?

Problems with shape and resolution of features

So we have to use layers!

Layers

Streams Cover

Field & Forest

Road

For streams and Roads sometimes the non-feature can have a value but is usually “no-data”.

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 RasterThe 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

Landuse

Streams

pipelines

Roads

Skewer of Location

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

• Counting cells

• Basic addressing has origin at upper left

0 1 2 3 4 5 6

1

2

3

4 4,6

E - W

N -

S

A raster can be up to 4 x 4 million grid cells

Addressing raster location – real world

• X,Y coordinates

• Origin at lower left

Increasing X (Easting)

Incr

easi

ng Y

(N

orth

ing)

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 1

Layer 2

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