1 / 24

Understanding Georeferencing for GIS Applications

Learn the importance of georeferencing in GIS, coordinate transformation methods like affine and polynomial, types of transformations, resampling, and practical examples. Gain insights into vector data types, topology, spatial queries, and attribute data organization in GIS.

harrison-glass
Download Presentation

Understanding Georeferencing for GIS Applications

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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!

More Related