Digitizing

• There are three primary methods for digitizing spatial information:

• Manual Methods include:• Tablet Digitizing• Heads-up Digitizing

• An Automated Method includes:• Scanning and Vectorization

Manual Digitizing

Ref: Getting Started with GIS, by K. Clarke

How is it done?

• Although methods vary, the typical steps for digitizing a paper map require the following:– Physical Preparation

• Digitizer Set up• Map preparation

– Digital Preparation• Map registration

– Digital Collection• Feature collection• Feature correction• Save, save, save

Creating New Data (Cont’d)

• Manual Digitizing:– Digitizing tables used (tedious process)– Electronic mesh senses cursor position– Paper maps (e.g. Topo Maps) are sources of most

GIS themes that we use– Point or stream mode digitizing done– Now-a-days mainly done in 3rd world– Need to record ref. coor. converted to real-world

coor. (GIS coverage)– Digitizing polygons need labels (ID)– Careful planning needed before you digitize

Heads up Digitizing

• Digitizing and scanning

Scanning

Ref: Getting Started with GIS, by K. Clarke

Creating New Data (Cont’d)

• Scanning:– Scan a map into binary grid file – Can be converted back to vector thru tracing (i.e.

line thinning extracting topo-reconstruction or smoothing)

– Software available to clean scanned images– Resolution is an issue (e.g. a line = 5-6 pixels)– Digital maps are as good as the source map

Creating New Data (Cont’d)

• On-Screen (head-up) Digitizing:– Relatively easy to do – Good for updating/editing changes– You can add points, lines, polygons– You don’t have to register points as in manual

digitizing

• Geometric Transformation:– Digitized maps have same coor. As source map

• You may have to convert to real-world coor. & projection using control points.

Raster to Vector Conversion•Able Software R2V

•AutoDesk Raster Design

•ESRI ArcScan

Filters

Rubber Sheeting: Spatial database editing software that attempts to correct errors by stretching a map to fit known control points or monuments.

Why Topology Matters

• Topology allows automated error detection and elimination

• Need to build/clean topology for digitized/scanned maps

• Unconnected nodes are snapped• Slivers from digitizing are eliminated• Topology makes map overlay feasible• GIS operations are done w/o accessing

the point files

Why do I want Topology?Gaps and slivers

Overshoots and undershoots

Non-Topology

Ref: Introduction to GIS by K. Chang

Creating New Data

• Remotely sensed data

• Georef; 1m x 1m resolution; good as a background map for other themes

Ref: Getting Started with GIS, by K. Clarke

Creating New Data (Cont’d)• GPS: differential GPS

have cm-scale resolution– Measures ranges

(distances of receivers by 24 NAVSTAR satellites using radio signals & atomic clocks)• Ref. or base stations can

reduce noise• Diff. GPS comes w/

software to download data• Hand-held GPS data can be

downloaded too

Ref: Getting Started with GIS, by K. Clarke

Creating New Data (Cont’d)

• Field Survey Data: – defines parcels by

azimuth & distances– Uses Total Station,

Stadia, Laser Ranging, and COGO software

Ref: Getting Started with GIS, by K. Clarke

Methods of Collecting Data

• Existing data, downloads from field

• Keyboard• COGO

Creating New Data (Cont’d)

• Street Addresses:– Point features, street

segment length, even/odd house numbers with side descriptions (e.g. TIGER files) can be geocoded

• TEXT Files: lat/long in dec. degrees cane converted to an event theme

Ref: Getting Started with GIS, by K. Clarke

Geocoding

What is Conflation?

• the process of transferring attributes from a source line layer based on one level of accuracy and precision to a target line layer of a different precision and accuracy.

Format & Exchanging of GIS Data

• Can import different data formats or use utility programs (e.g Import 71) to convert data

• GIS accept TIF, GIF, JPEG, AutoCAD data formats (needs right extensions) which are not georeferenced

• Vector to Raster conversion is easy, opposite is hard• Data exchange by import/export involves errors in

attributes & geometry• ESRI New Product called Data Interoperability • Efficient data transfer is important for the future of GIS

– OpenGIS® Specifications on• GML Geographic Markup Language• WMS Web Map Service• WFS Web Feature Service