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 ( –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