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Intro. To GIS Midterm Review March 8 th , 2013

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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Intro. To GIS Midterm Review March 8 th , 2013

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  1. Intro. To GISMidtermReviewMarch 8th, 2013

  2. Reminders • Lab on next Monday • Try to catch up on homework assignments

  3. Why Georeferencing?

  4. 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

  5. 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,…)<<

  6. 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

  7. 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

  8. 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

  9. A bit of clarification on Optical RS The end result is surface reflectance/temperature or a thematic map (classified map)

  10. Orthophoto Vs. Aerial photos (or Remotely sensed Imagery)

  11. Orthophoto Vs. Aerial photos (or Remotely sensed Imagery)

  12. 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

  13. 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

  14. Topology • Concepts • Adjacency • Enclosure • Connectivity • Terms to be defined • Node • Arc • Polygon

  15. 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?

  16. Organizing Attribute Data • Flat Files • Spreadsheets (e.g. excel spreadsheet)

  17. Organizing Attribute Data • Hierarchical

  18. Organizing Attribute Data • Relational (What is commonly used in GIS) • Various tables (databases) are “linked” through unique identifiers

  19. 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

  20. 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

  21. Selection Criteria (#8.8) Per capita energy use > 4,000 AND population < 20,000,000

  22. Selection Criteria (#8.8) [Per capita energy use < 4,000 OR (population > 40,000,000)] AND (car theft <1)

  23. Selection Criteria (#8.8) Population < 20,000,000 OR car theft > 1.5

  24. Good luck on your midterm!

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