EARTH309 Exam 2

Each pixel in a raster model contains…

One numeric value representing some property of that pixel

Either integer or floating point

The precision of a raster is limited by the ___

Resolution

Attribute tables are created for discrete rasters with a limited number of unique values. What are the value and count fields?

Value: contains each unique value

Count: indicates how many cells have that value

Data that are in raster GIS format:

Digital Elevation Model (DEM): continuous

Color aerial photograph: continuous

Land use categories: discrete

Primary, secondary, and primitive roads: discrete

Scanned and georeferenced topography map, aka Digital Raster Graphic: continuous

ArcGIS Online basemap raster tile

Data vs Picture rasters

Data Rasters: store values representing measurements or categories and can be analyzed

Picture Rasters: store color values and cannot be analyzed (like taking a picture of a map)

Tiled Rasters

Map is divided into equal squares, a raster snapshot is recorded for each square

Transmits and displays complex maps quickly

Useful for background display (basemaps, DRG topographic maps)

Multiband Rasters

Contain multiple raster arrays with the same number of rows and columns and the same resolution, stored together in a multiband stack

Common examples: satellite images, color aerial photographs

Voxels

Raster cell with height, encompassing a volume

Voxels are stacked to store data values in 3DG

Georeferencing (definition)

Identifies pairs of ground control points visible in the raster and in a reference dataset, the calculates real-world coordinates from the control point pairs and applies it to the raster

Georeferencing (how to do it)

1. Add reference data (data that you want the image to line up to, usually a basemap or basic states/counties polygon layer)

2. Add image

3. Check data frame coordinate system and use the image’s projection

4. Click “fit to display”

5. Control points - click on image first, then on reference data

   1. Update georeferencing - adds files with CS information

Export Raster tool

Makes a copy of a dataset or a subset of the features

Build Pyramids

Improves display performance of rasters, resamples and compresses data

Successively lower-resolution copies are created by representing 4 adjacent cells as a single pixel → increases storage size of dataset (but does not permanently change the data)

Resampling

Changes resolution of raster

Required when projecting rasters or analyzing with different resolutions

Nearest Neighbor Resampling: uses value from the old cell that falls in the center of the new cell, most common way to resample

Clipping Rasters

Clip tool cuts out a portion of a raster dataset

Clipped area is the extent of the map display, or specify a rectangular minimum and maximum x-y coordinate

Merge tool

Creates a single item out of multiple rastersM

Mosaic Dataset tool

2 or more adjacent raster datasets are merged into one entity

Can minimize abrupt changes along boundaries of overlapping rasters