D IGITIZING AND S CANNING

1. DIGITIZING AND SCANNING OUTLINE: geocoding digitizing terms and methods scanning methods adding attributes

2. DIGITIZING AND SCANNING • we can buy data, steal it, download it over the internet or collect it ourselves

3. GEOCODING DATA • conversion of spatial information into digital form • involves capturing the map, and sometimes also capturing the attributes • often involves address matching • method can influence the structure and error associated with the spatial information which results

4. GEOCODING DATA • digitizing • scanning • field data collection (remote sensing, geographic databases, field work)

5. GEOMATIC DATA Two aspects of geodata: • Spatial: points, lines, and polygons that are referenced to a position on the earth • Attribute: characteristics of the spatial data

6. DIGITIZING TERMS

7. DIGITIZING AND SCANNING • convert map (analog) data to digital data. • collecting the geometry, attributes, topology. • establishing a geographic frame of reference.

8. VECTOR/MANUAL DIGITIZING • captures map data by tracing lines from a map by hand • components: • electronically-sensitive tablet or digitizing surface (sizes 12" x  18"  ->  36" x  48"). • curser with crosshairs and keypad • computer interface

9. VECTOR/MANUAL DIGITIZING

10. VECTOR/MANUAL DIGITIZING • trace paper map line by line (vertices and nodes) • one layer at a time • if you have a map with roads, parks, building – do all roads first • control or reference points (3-6) – help line up work from day to day, layer to layer

11. VECTOR/MANUAL DIGITIZING Vector digitizing steps: • fix to tablet • digitize control • determine coordinate transformation • trace features • proof plot • edit • clean and build (code and add attributes)

12. VECTOR/MANUAL DIGITIZING • smoothness of lines depends on number of vertices created

13. DIGITIZING TABLE • no point in having a table that is much more precise than an operator can point to • digitizing tables need to be calibrated regularly • if map is larger than table, will have problems trying to take the digitized pieces and knit them together

14. VECTOR DIGITIZING Point mode:mouse used to record points, points are digitized by placing the cross hairs over the point one wishes to digitize and pressing the button. • record point features • minimizing the size of the data file • accuracy mostly controlled by skill of operators and line weights

15. VECTOR DIGITIZING Line mode: (automated mode) – points are recorded at a fixed rate • straight lines usually have few points recorded. Complex areas more points • more operator time, data storage capability and computer processing time

16. PROBLEMS • labour intensive • operator fatigue; requirements for accuracy and errors. • topology requirements force extra specifications • sheet matching and registration. • specialized equipment

17. DIGITIZING ERRORS • Missing line

18. DIGITIZING ERRORS • Undershoot

19. DIGITIZING ERRORS • Overshoot

20. DIGITIZING ERRORS • Multiple Identifies (attributes) • associated with undershoots or missing lines

21. DIGITIZING ERRORS • No Identifiers • from simple omission

22. DIGITIZING ERRORS • Extraneous Linework • creates unlabelled polygons

23. DIGITIZING CONTROLS • a lot of visual inspection • use software to verify relationships expected to occur • attributes verified by other relationships or brute force inspection.

24. DIGITIZING CONTROLS • Fuzzy tolerance - minimum distance between all (nodes and vertices) arc coordinates. • typical minimum distance between coordinates .002 inches. • handles small overshoots or undershoots, automatic sliver removal, and coordinate thinning of arcs. • exercise caution when using the fuzzy tolerance

25. DIGITIZING CONTROLS Dangle Length - minimum distance between arc coordinates. • a dangling arc has the same polygon on its left and right side. Node match tolerance(snap tolerance) - minimum distance between node features. Weed tolerance - distance between coordinates (vertices) within each arc

26. HEADS-UP DIGITIZING • looking at monitor • creating new maps by editing existing layers • create zip codes from blocks, create bus line from streets • caution – new maps only as accurate as original • precision dependent on • resolution of the screen (at normal zoom) • resolution of the data on the screen • how easily the operator can point to things.

27. SCANNING • device that converts maps into digital form • originally single sensor on a drum • uses CCD arrays to record information in binary form (a semi-conductor that translates photons into counts of electrons)

28. SCANNING

29. SCANNING • places a map on a glass plate, and passes a light beam over it • measures the reflected light intensity • result is a grid of pixels • image size and resolution are important • use software to clean up image and transform it from image to data set

30. SCANNING EXAMPLE

31. SCANNING • caution – scanner will include the whole map, not individual layers (also include coffee stains, map folds, pencil marks…) • individual layers must be re-traced in computer or extra layers must be digitally erased (cleaned)

32. ADDING ATTRIBUTES • simple attributes – can be coded at time of digitizing • cursor buttons or menu • extensive attributes – held in a relational database management system (RDMS) • attach feature codes to graphic entities. • attach unique identifiers then enter no-spatial data afterwards.

33. AVOIDING DIGITIZING/SCANNING • you can avoid digitizing by finding a source for the digital database "somewhere" out there. • remote sensing or geographical databases