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Creating and editing spatial data RESM 440 Lecture 12. Today. Tests: All have been graded, will be handed back next week after everyone takes the exam Grades on ecampus now for everyone Topic: Creating and editing spatial data This week in lab: Using imagery, digitizing

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Presentation Transcript
  • Tests:
    • All have been graded, will be handed back next week after everyone takes the exam
    • Grades on ecampus now for everyone
  • Topic:
    • Creating and editing spatial data
  • This week in lab:
    • Using imagery, digitizing
  • Extra reading: Bolstad, Chapter 4
  • Review: Where spatial data comes from
  • Editing vector datasets: Digitizing
    • Forms of digitizing
    • Digitizing and accuracy
    • Specifics of digitizing
  • Preparing source maps
  • Sources for spatial data
    • Other methods already covered
      • Obtaining existing datasets
      • Image classification (remote sensing)
    • Turning information into maps
      • GPS coordinates
      • Tabular joins to existing data
    • Digitizing your own data
      • Source maps: Aerial photos, paper maps
gps overview
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
collecting gps data
Collecting GPS data
  • Not just humans…

Remotely operated vehicle (underwater)

Tracking collars on animals

using gps data










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







preparing gps data for use in gis
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


Conversion Formula










Remember to convert X to negative (after calculation)

preparing gps data for use in gis steps
Preparing GPS data for use in GIS: Steps
  • Convert data to decimal degrees in Excel
  • Save Excel spreadsheet in correct file format
    • Make sure 1st row includes column headings
    • Column headings cannot include spaces, must be unique
in gis software steps
In GIS software: Steps
  • In ArcGIS: Display XY data menu option
    • Automatically maps locations as points using X,Y coordinates from table

Fish Sampling Points

Pymatuning Lake, PA

WVU Wildlife Mgmt Class (2007)

  • 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
forms of digitizing
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
forms of digitizing1
Forms of digitizing



On Screen

the process of digitizing on screen



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

Morgantown North


  • For each quad:
  • Aerial photos
    • DOQQs (late 1990s)
    • SAMB (2003)
    • NAIP (2007, 2009, 2011)
  • Scanned topo maps (DRGs)
the process of digitizing on screen1
The process of digitizing: On-screen
  • Create new output dataset (e.g. shapefile)
    • Specify file name, feature type
      • Point, line, polygon
    • Specify projection (i.e. UTM)
    • Attribute table fields
    • Easily done in Catalog window
step 3 create new features

Double-click to end

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

Double-click to end

Polygon automatically closes itself




the process of digitizing on screen2
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

Football Field

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


digitizing and accuracy
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 more detailed map scales
    • Operator errors: going too fast, fatigue, strained eyesight etc.
editing attribute values review
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
georeferencing images
Georeferencing images
  • Georeferencing: Image registration
  • Process of aligning a spatial dataset with its correct location in a coordinate system

Scanned Mine Map

process of georeferencing image
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)
using control points
Using control points
  • Control points are used to register dataset to known location
  • Use:
    • Street intersections, corners
    • Buildings
    • Other landmarks that do not move


Parcel Map (Unknown)

Aerial Photo


good control points
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
  • Review: Where do GIS datasets come from?
    • GPS
    • Existing data
    • Digitized data
  • Heads up digitizing
  • Editing features
  • Editing attribute tables
  • Georeferencing images