Data input 1:- Online data sources

-Map scanning and digitizing

GIS 4103 Spring 06Adina 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