Lecture 6: Raster GIS Analysis

How do we encode a tessellated grid?

• Horizontal locations (X,Y) controlled, height Z (attribute) measured

• Systematic sequence of points where height is measured

• Profiles: straight lines with regular sampling along them

• Relatively easy to operate this representation, but it’s redundant in smooth areas, loses details in rough terrain, has variable accuracy

Raster Resolution is limited by ____

pixel size

What is resolution?

• Resolution determines how accurate the data is, how well features are defined, and how detailed the representation becomes.

• Higher resolution = more detail, more accuracy, better feature definition.
  Lower resolution = less detail, more generalization, potential loss of important features.

Integer Point Grid

Use integer grids to represent discrete data (land use classes)

attribute table

Floating Point Grid

Use floating-point grids to represent continuous data (elevation)

Surface

• A surface is a continuous representation of a geographic variable (like elevation).
  It shows how a value changes across space.

Aspect

Direction a slope faces

Volume

amount of space between a surface and a reference plane.

Viewshed

A viewshed shows what areas are visible from a specific point. It maps where you can see

Interpolation

• Interpolation estimates values at locations where you don’t have data by using the values you do have.

• It creates a continuous surface from scattered points.

Raster Overlay aka Grid Overlay

• Everything represented by cells

  • Point – Single Cell

  • Line – String of Cells

  • Area – Group of Cells

• With raster overlay, layers can be added, subtracted, multiplied or divided.

• Two issues need to be considered during this process:

  • resolution

  • measurements scales.

Vector Overlay aka Polygon Overlay

• Vector overlay combines layers using geometry.
  It uses operations like intersection, union, clip, erase, based on points, lines, and polygons.
  Overlay = geometry + attributes.

• Example:
  Intersect parcels with flood zones → output polygons with combined attributes

Point-based

center point (regular grid) -DEMs, - but what if periodicity in landscape?; what if pop. density?

systematic unaligned (random in a cell)

area-based

• extreme value (max or min)

• total (sum, like reflected light)

• predominant type (most common)

• presence/absence (binary result)

• percent cover (% covered by single category)

• precedence of types (highest ranking)

How can we represent surface?

1. Grid or Raster

2. TIN (Triangulated Irregular Network)

3. Contour and flowline (mesh)

How can the government improve our quality of life?

What is an indicator?

healthcare, poverty. education…

What is measurable data?

specific, what is the average percentage of college enrollment?

Scale of analysis

Country, state, county, neighborhood, block