Data Storage & Editing

1. Data Storage & Editing GEOG370 Instructor: Christine Erlien

2. Overview • Storing GIS databases – tiling • Types of error that can occur • Entity errors • Attribute errors • Projection and error • Edge matching • Conflation

3. Data Storage & Editing Subsystem • Importance • Tools to store & maintain data • Tools to perform QA/QC on data  pinpointing errors • Need to be aware of possible errors to know what to look for in databases you are working with

4. Data Storage & Editing Subsystem • Types of errors: • Entity error: Error in position • Attribute error: Incorrect attribution • Entity-attribute agreement error: Codes associated with wrong entities

5. Data Storage & Editing • Before beginning a spatial analysis or mapping project  check data • Error detection and correction may include: • Compare data with input document • Check topology of spatial objects • Check attributes of spatial objects • Check for missing spatial objects

6. GIS Database Storage Raster • Data • Attribute values for grid cells • Associated tables • Editing • Concerned with verification correct positions of grid cells • Examine column & row position, attribute code

7. GIS Database Storage Vector • Data • Individual tables within a single database • Different databases, linked by pointers • Editing • Entities • Missing objects, incorrectly placed objects, incomplete links • Attributes • Typos, incorrect codes, attributes incorrectly associated with a field • Entities & attributes together • Codes associated with incorrect entities

8. Vector Data Storage & Editing Tiling—storing database in subsections • Reduces computational overhead, speeds up analysis by decreasing data volume • Greater control of editing process From Fundamentals of Geographic Information Systems, Demers (2005)

9. The importance of editing the GIS database • Sources for error: • Hardware (digitizing, scanning) • People • Digitizing • Attribute inputs

10. Detecting & Editing Errors: Vector Identifying entity errors • Build topology • Look at database statistics Check that: • All entities that should have been entered are present • No extra entities have been digitized • Entities are in the right place & of correct shape/size • Topology is correct • Polygons have only a single label point • Entities are within the tic mark boundary

11. Entity Errors: Vector Pseudo nodes: False nodes; where a line connects with itself or where two lines intersect along a parallel path rather than crossing. From Fundamentals of Geographic Information Systems, Demers (2005)

12. Entity Errors: Vector • Dangling node (dangle): Node connected to a single line entity (doesn’t indicate intersection) • Produced by: • Failure to close a polygon • Failure to connect the node to appropriate object (undershoot) • Going beyond the entity the node should be connected (overshoot)

13. Entity Errors: Vector Undershoot Overshoot From Fundamentals of Geographic Information Systems, Demers (2005)

14. Entity errors: Vector Polygon digitizing • Polygon requires point inside that will act as a locator for a label • Possible errors: • Missing labels • Too many labels • Generally caused by failure to keep track of the digitizing process

15. Entity errors: Vector From Fundamentals of Geographic Information Systems, Demers (2005)

16. Entity errors: Vector • Sliver polygons: Small polygons produced by digitizing adjacent lines between polygons > once • Occur when digitizing software uses a vector model that treats each polygon as a separate entity • This type of software becoming less common • Fix: Compare # of polygons from input map to digital coverage; zoom in to suspect areas http://www.ncgia.ucsb.edu/cctp/units/unit28/28.html

17. Entity errors: Vector • Weird polygons: Polygons with missing nodes • Cause: Point digitized in wrong place or wrong order • Prevention: Map preparation or digitizing method • Fix: Moving lines to correct locations http://www-users.aston.ac.uk/~elgyj/data_structures.htm

18. Entity & Attribute Errors: Vector • After making changes: • Rebuild topology • Changes modified some spatial relationships  topology needs to be updated • Save!! • Incorrect attributes • Prevention: Keep track of attributes as typing in • Fix: Correct them! Save!

19. Attribute Errors: Raster Raster: • Missing attributes: Missing entire (or portion of) row or column • Incorrect or misplaced attributes • Attributes errors occurring along area margins cells may be reassigned to neighboring polygon

20. From Fundamentals of Geographic Information Systems, Demers (2005)

21. Dealing with Projection Changes • Coordinate conversion as editing • Cartesian (digitizer)  real-world • Projection produced using transformations: • Scale • Rotation • Translation • Some error: Mathematical process of projection & computer rounding • Look at software’s measure of error  if high, there may be problems with digitizing

22. Edge Matching Vector • Linking adjacent maps (usually the same theme) to permit analysis of larger study area • Potential difficulties: • Same projection, but digitized separately  mismatches • Different projection or same projection, different datum • Fix: Link line & polygon entities that should be connected

23. Edge Matching Raster • Remote sensing products • Horizontally adjacent scenes collected at different times  latitudinal coordinates may differ slightly • Skew between images • Correctable by shifting grid until matches

24. Conflation & Rubber Sheeting • Conflation: Process of integrating map data from multiple sources • Generally requires rubber sheeting • Rubber sheeting/Warping • In a reference map, features for which coordinates known are identified (control points) • Goal: Additional layers made to conform to reference layer using control points

25. From An Introduction to Geographical Information Systems, Heywood et al. (2002)

26. Wrapping up: You should know • What tiling is & its purpose • 3 basic types of error & how to edit them • entity, attribute, entity-attribute • Types of entity error • All entities that should have been entered are present • No extra entities have been digitized • Entities are in the right place & of correct shape/size • Topology is correct • Polygons have only a single label point • Entities are within the tic mark boundary

27. Wrapping up: You should know • Types of raster & vector attribute error • How they occur • How detected & corrected • How projection introduces error • The purpose of edge matching • The purpose & process of edge matching