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Intro. To GIS Midterm Review March 8 th , 2013. Reminders. Lab on next Monday Try to catch up on homework assignments. Why Georeferencing?. Georeferencing. Georeferencing

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Presentation Transcript
reminders
Reminders
  • Lab on next Monday
  • Try to catch up on homework assignments
georeferencing
Georeferencing
  • Georeferencing
    • The process of converting a map or an image (or scanned map) from one coordinate system to another by using a set of control points and a transformation equation.
  • Two steps
    • Coordinate transformation (scaling, rotating, skew)
    • Resamping
coordinate transformation
Coordinate Transformation
  • Methods
    • First-order polynomial (Affine)
    • 2nd Order polynomial
    • 3rd order polynomial
    • Spline

1st order

2nd order

>>Control points are used to estimate the coefficients (a0,b0,…)<<

transformation types affine
Transformation types: Affine
  • The affine transformation function is:
  • x’ = Ax + By + Cy’ = Dx + Ey + F
    • where x and y are coordinates of the input layer and x’ and y’ are the transformed coordinates.
    • The C and F parameters control shift in origin (translation)
    • A, B, D, E control scale and rotation
    • their values are determined by comparing the location of source and destination control points.
  • Scales, skews, rotates, and translates
  • 6 unknowns( A,B,C,D,E,F) so a minimum of three “displacement links” required
  • Little benefit from more than 18-30 links
  • The most common choice
example transformation
Example: Transformation
  • Let’s do a simple example
    • We would like to calculate new coordinates for point A(x=1, y=1), i.e., we want to convert coordinate system (x,y) to (x’,y’).
    • We assume a 1st order (affine) transformation works fine
    • All the six coefficients (for affine transformation) are given (a0=1, a1=1.1, a2=0.4 and b0=0.2,b1=1.8,b2=0.8)
    • x’ and y’ are the new coordinates for (x,y) in the new coordinate system
    • Continue on next Slide >>>>

1

.5 , 8

resampling
Resampling
  • Let’s continue on… After the transformation, the question is:
    • What is the pixel value for .5 , 8???? (That’s what we call resampling)
  • The new coordinate system is, in fact, a new raster dataset (right), which is slightly rotated, scaled, skewed, or distorted depending on the order of polynomial.
  • We need to estimate pixel values from the original raster data (left/yellow dot), i.e., resampling, for the new dataset (right/green)

y’

y

2

3

3

1

2

1

73

78

78

2

70

74

80

1

3

68

69

65

coordinate

x

1

e.g., Average of 80 and 68 would be the pixel’s new value

Pixel value

2

x’

3

a bit of clarification on optical rs
A bit of clarification on Optical RS

The end result is surface reflectance/temperature or a thematic map (classified map)

midterm overview
Midterm Overview
  • Based primarily on lecture and homework/book
  • Good knowledge of lab exercises helps!
  • Closed notes, closed book, no computers
  • Basic calculators
  • Question types will include:
    • Multiple choice
    • True-False
    • Short answer
    • Few long answer
vector data and topology
Vector Data and Topology
  • Topology
    • The arrangement for how point, line, and polygon features share geometry
    • Or knowledge about relative spatial positioning
  • Two types of vector models exist in a GIS
    • Geo-relational Vector Model
      • Arc Coverage (has topology) >>> format: binay
      • Shape files (no topology) >>>> format: *.shp, *.shx, *dbf, etc.
    • Object-based Vector Model
      • Includes classes and geodatabases >>> format: *.mdb
topology
Topology
  • Concepts
    • Adjacency
    • Enclosure
    • Connectivity
  • Terms to be defined
    • Node
    • Arc
    • Polygon
query
Query
    • A query is a “question” posed to a database (attribute data)
  • Examples:
    • Mouse click on a map symbol (e.g. road) may mean
      • What is the name of road pointed to by mouse cursor ?
    • Typing a keyword in a search engine (e.g. google, yahoo) means
      • Which documents on web contain given keywords?
    • SELECT ‘FROM Senator S’ WHERE S.gender = ‘F’ means
      • Which senators are female?
organizing attribute data
Organizing Attribute Data
  • Flat Files
    • Spreadsheets (e.g. excel spreadsheet)
slide18

Organizing Attribute Data

  • Relational (What is commonly used in GIS)
    • Various tables (databases) are “linked” through unique identifiers
query making selections
Query: Making Selections
  • Usually interested in some subset of the data
  • Selections can be made in two primary ways:
    • Select by Attribute – specify matching criteria
    • Select by Location – based on spatial proximity
select by attribute tips
Select by Attribute Tips
  • Be careful with case sensitivity and spaces
  • Use parentheses to carefully construct a query
  • Use “Boolean” Operators (AND, OR, NOT, LIKE)
    • AND means both criteria, OR means either
    • NOT allows you to exclude some criteria
    • LIKE lets you be more flexible, use wildcard characters (_ for one character, % for many)
    • Verify your expression to make sure it works
selection criteria 8 8
Selection Criteria (#8.8)

Per capita energy use > 4,000 AND population < 20,000,000

selection criteria 8 81
Selection Criteria (#8.8)

[Per capita energy use < 4,000 OR (population > 40,000,000)] AND (car theft <1)

selection criteria 8 82
Selection Criteria (#8.8)

Population < 20,000,000 OR car theft > 1.5