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Creating and editing spatial data RESM 440 Monday October 4, 2010

2. Today. TestsTopic:Creating and editing spatial dataThis week in lab:Using imagery, digitizingExtra reading: Bolstad, Chapter 4. 3. Overview. Review: Where spatial data comes fromEditing vector datasets: DigitizingForms of digitizingDigitizing and accuracySpecifics of digitizingPrepar

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Creating and editing spatial data RESM 440 Monday October 4, 2010

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    1. Creating and editing spatial data RESM 440 Monday October 4, 2010

    2. 2 Today Tests Topic: Creating and editing spatial data This week in lab: Using imagery, digitizing Extra reading: Bolstad, Chapter 4

    3. 3 Overview Review: Where spatial data comes from Editing vector datasets: Digitizing Forms of digitizing Digitizing and accuracy Specifics of digitizing Preparing source maps

    4. 4 Review

    5. 5 GPS Overview GPS = Global Positioning Systems Used to collect locations in the field Network of 20+ satellites in orbit GPS receivers use signals from satellites to determine location Accuracy varies Few cm ($$$$$) to Several meters ($) Data collected may be used in GIS

    6. 6 Collecting GPS data Not just humans…

    7. 7 Using GPS data GPS coordinates give longitude (X) and latitude (Y) Usually saved in a table (Excel)

    8. 8 Preparing GPS data for use in GIS GPS “waypoint” X, Y coordinates may need to be converted for use in GIS, use Excel to convert data using formula below GIS requires a single X, single Y value Most commonly used: Decimal degrees Degrees: units of measurement in lat/long system on the globe 60 “minutes” in one degree of longitude or latitude 60 “seconds” in one minute

    9. 9 Preparing GPS data for use in GIS: Steps Convert data to decimal degrees in Excel Save Excel spreadsheet in correct file format (txt, dbf) Make sure 1st row includes column headings Column headings cannot include spaces, must be unique

    10. 10 In GIS software: Steps In ArcGIS: Add XY data menu option (Tools) Automatically maps locations as points using X,Y coordinates from table

    11. 11 Digitizing Types of map data sources: Hardcopy: refers to hand-drawn or printed paper maps Digital: refers to computerized information Digitizing is the process of collecting digital spatial coordinates = data entry for GIS… Data collection is very significant part of any GIS project

    12. 12 Forms of digitizing Manual Input data source: Paper map Uses digitizing table, light table Users move “puck” over map, click to trace features Heads-up or on-screen Input data source: Scanned map or image Move & click mouse to trace features on the screen Automated Input data source: Scanned map Computer software “recognizes” features on the map

    13. 13 Forms of digitizing

    14. 14 The process of digitizing: On-screen Obtain source dataset(s)

    15. 15 The process of digitizing: On-screen Create new output dataset (e.g. shapefile) Specify file name, feature type Specify projection Attribute table fields Usually done in ArcCatalog

    16. 16 Step 3. Create new features Digitizing: Points Lines Polygons Nodes: Starting and ending points of lines Vertices: Internal XY points in lines

    17. 17 The process of digitizing: On-screen Start editing output dataset Create new features: Trace outlines of features Edit features and attributes Correct mistakes Edit table: Labels, etc. Save your work

    18. 18 Editing features Many options exist to make your digitized features more neat and precise: Copy/paste Split Merge Digitize using exact angle, direction, or distance Add new polygon with shared border

    19. 19 Snapping Common errors in digitizing: Dangles Overshoots Slivers along borders Snapping A method to make sure your vertices exactly match up

    20. 20 Digitizing and accuracy What affects accuracy of digitized data? Humans interpret source maps as they go along – mistakes in interpretation Precision of equipment being used: Limit to how close points can be to one another Errors become magnified at smaller map scales Operator errors: going too fast, fatigue, strained eyesight etc.

    21. 21 Editing attribute values (review) Adding a field to a table Editing values in a table Type in new data “by hand” Calculating geometry (area, length, perimeter, X, Y) Calculating field values

    22. 22 Georeferencing images Georeferencing: Image registration Process of aligning a spatial dataset with its correct location in a coordinate system

    23. 23 Process of georeferencing image Add image to map (unknown location) Add reference data to map (known location) Find control points Check error (RMSE) Save georeferencing (“rubbersheets” the image)

    24. 24 Using control points Control points are used to register dataset to known location Use: Street intersections, corners Buildings Other landmarks that do not move

    25. 25 Good control points Criteria for control points: Spread evenly around study area Minimum number of points (at least 4) Use highest accuracy dataset possible Use locations identifiable on both datasets (known & unknown) Final result: Your image is saved with new georeferencing

    26. 26 Summary Review: Where do GIS datasets come from? GPS Existing data Digitized data Heads up digitizing Editing features Editing attribute tables Georeferencing images

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