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NR 322: Editing Spatial Data
Jim Graham
Fall 2008
Chapter 6
Digitizing Spatial Data
• We are creating a representation of objects on the earth
• What accuracy is important?
• How did we determine their position?
• How will the data be used?
Accuracy
• Accuracy: How closely the data matches truth
• What accuracy do we need to maintain?
• Typically you want to collect data that is twice as accurate as will be needed to allow for conversions to decrease the accuracy.
Data Collection
• How was the data collected?– GPS– Topographic Map– Surveys to benchmarks– “About here” on a satellite photo
• Each step has an effect on accuracy:– Instrument– Digitizing– Transforms (projection)
How will the data be used?
• Maps at large extents, low resolution?
• Maps at small extents, high resolution?
• Analysis?
Digitizing Process
• ArcCatalog:– Create the Shapefile
• ArcMap:– Add the Shapefile (or Coverage, or GeoDatabase)– Add attribute fields– Start Edit– Add points, polylines, polygons– Add attribute values– End Edit
• ArcCatalog:– Add Metadata
Create New Shapefile
• In ArcCatalog:– Select New -> Shapefile– Select the type of file– Define the spatial reference (coordinate
system) to match the raster– Z adds an elevation– Don’t worry about M yet
Questions to ask before editing
• Points:– What part of the feature to identify?
• Adding Polylines– What level of stream?– What level of road?
• Adding Polygons– What level of resolution?– How small a feature to include?
Editing Points
• Adding:– Tool: Sketch Tool– Task: “Create New Feature”– Target: Matches Point Layer– Click to add points
• Edit/Delete:– Tool: Edit Tool– Task: NA– Drag to move points– Right-click to delete points
Editing Polylines
• Adding Polylines:– Set “Snapping”– Tool: Sketch Tool– Task: “Create New Feature”– Click to add points– Double-click to end polyline
• Editing Polylines:– Tool: Edit Tool– Task: Modify Features (watch this)– Drag to move vertices with special cursor– Right-click to delete vertices
Editing Polygons
• Adding Polygons:– Set “Snapping”– Tool: Sketch Tool– Task: “Create New Feature”– Click to add points– Double-click on start vertex
• Editing Polylines:– Tool: Edit Tool– Task: Modify Features (watch this)– Drag to move vertices with special cursor– Right-click to delete vertices
Editing Metadata
• Select the file in ArcCatalog
• Select “Edit metadata”
• Enter all 200 fields!
• Fill out:– Identification: required fields– Data Quality– Spatial Reference– (not required for Lab 7!)