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 StreamsCover 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 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 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 addressi ng has origin at upper left ,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) Increasing Y (Northing)

In ArcGIS In ArcGIS most rasters are GRIDS There are two (2) basic types of Grids –Integer -Discrete data, Data range to –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