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Data input 1:- Online data sources
-Map scanning and digitizing
GIS 4103 Spring 06
Adina Racoviteanu
Data entry
= operation of encoding data for inclusion into a database
- time-consuming part of GIS!
Methods of data entry
1. transfer of data from existing digital sources
2. manual digitizing and scanning of maps
3. image data input and conversion to a GIS(aerial photography, satellite imagery)
4. direct data entry: GPS
1. Transfer data from existing digital sources
• Online data sources?– ESRI's web page (http://www.esri.com) – GIS data depot– Etc etc…
Issues with transferring data: data quality
• meta-data = data about the data – How old are the data? – Where did they come from? – Areal coverage? – Map scale? – Projection, coordinate system, and datum? – How accurate are positional and attribute features? – Is the data relevant to the project at hand? – Format of the data? – Has the data been checked? – Why was the data compiled? – What is the reliability of the provider?
Issues with transferring data:data formats
Common data transfer formats include:
• USGS's digital line graphs (DLG) – coding information drawn from the USGS's
conventional paper quadrangle sheet maps.
• Spatial Data Transfer Standard (SDTS)
• JPEG files
Common data formats
• ArcInfo – .e00 - Import/Export format – .shp, .shx, .dbf – Shapefiles
• Digital Line Graphs (DLG)
– USGS vector data transfer format
• Spatial Data Transfer System (SDTS)- U.S. Government transfer format
• Digital Elevation Model (.DEM) – USGS elevation data in raster form
Common data formats
• Digital Raster Graphic (DRG) – digital USGS topographic Maps
• Tiger (Topologically Integrated Geographic Encoding and Referencing Files)- US Census Bureau base map data
2. Digitizing
= transformation of information from paper
map to digital format, so that it can be
stored and displayed with a computer
• manual
• semi-automated (automatically recorded while manually following a line),
• fully automated (line following).
Manual digitizing
• tracing map features in the form of points, lines or polygons with a mouse (puck)
• coordinate of each sample point stored in the computer
Tools:- digitizing table (or tablet): (heads-down digitizing).
digitizing table has a fine grid of wires embedded in it that acts as a Cartesian coordinate system
- OR: computer screen (heads-up digitizing).
Two modes of digitizing
• point-mode
• stream-mode
You select and encode those points deemed "critical" to represent the geomorphology of the line or significant coordinate pairs.
digitizing device automatically selects points on a distance or time parameter, which generates sometimes an unnecessary high density of coordinate pairs
Before starting…• For what purpose will the data be used? • What coordinate system will be used for the project • What is the accuracy of the layers to be associated? If it is significantly
different, the layers may not match. • What is the accuracy of the map being used?• Will it be merged with a larger database?
A few pieces of advice…
• Each time you digitize, digitize as much as possible.
• For more consistency, only one person should work on a given digitizing project
• If the source consists of multiple maps, select common reference points that
• coincide on all connecting sheets. Failure to do this could result in digitized
• data from different data sheets not matching.
• If possible, include attributes while digitizing, as this will save time later.
Post-digitizing steps
• Transform coordinates from the digitizer system to the real world system (e.g. national map grid)
• Correcting digitizing errors (undershoots, overshoots, triangles)
• Entering missing data, building topology
Scanning maps
• Documents must be clean
• Lines at least 0.1mm wide
• Automatic feature recognition
• Contour lines cannot be broken with text