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!)