User:Mav/Introduction to GIS notes by maveric149/2002-02-26 Lecture

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Introduction to GIS notes by maveric149 2002-02-26 Lecture

Database construction

  • Most expensive and time consuming part of any GIS project. (80% of time and money)
  • Prior to generating new data, look for previously created data.

Finding existing GIS data

  • knowledge, experience and luck
  • Internet ; government or public agencies (may not be stable source, not right format, not in digital or correct form.)

Publicly available GIS data

  • Federal Geogrpahic data Commitee
  • USGS -- major provider of GIS data, Digital Line Grpas, vector representation of the point line and area features of US
  • US Bureau of the Census. TIGER files, -- also has geocodable address indices for streets in most urban area of the country.
  • States, counties municipalities and special area (SACOG)

Proprietary GIS data

  • Generally, this the most accurate data is found here - slightly out of date data can be obtained for a minimal fee.
  • ESRI is a good source for links


  • info about spatial data
  • dry, but essential to let peole know where data came form
  • ID, title, contents, currency

Gualtiy information completenness Organization info - raster or vector
Reference info
Attribute info
Distribution info
Metadata info

Conversin of acquired existing data

  • data conversion
  • Direct translation (from one format to another)
  • Only a limited number fof fiters are available
  • conversion may also be done through use of neutral formats
  • Public agencies often do this
  • Also some private providers

Creating new data

  • Remote sensing data
  • most data are in raster data structure but can be use to make vector data input
  • Digital orthophoto quads (DOQs) are photgraphic images wih resolution of 1 meter that correspond to USGS 7.6 minute quod.

Creating new data

  • GPS data -- uses satellites to triangulate horizontal and vertical locations
  • with correction, accuracy in 3 to 5 meters

Digitizing table

  • use of digitizing table - contains analog to digital data
  • Discrete features are generally suitable for digitization and the process results in creation of vector data file.
  • use of a puck to click where analog map coordinates are.
  • Accuracy is generally within .0001 inches which is called as rmsi error in Arc info of ESRI Package - The full form of Rmsi error is root mean square error - Edited by A.Narayana Murthy.

Headline text[edit]

  • "Control points"

Two mode:

  • Point mode, operator slecis discrete point
  • stream mode, operator traces lines to be digitized and the location of the puck is placed at set intervals


  • creates a raster files form analog map
  • can be converted to vector by sectorization
  • use of "tracing" (cheap and automated)

Creating New Data

  • on screen digitizing is amhula
  • used when feature are preson (GAP)

Importance of source map accuracy

  • paper maps tend to shrink or expand as temperature and humidity change. The age rather vintage of the maps also contributes alot to the data that have been digitized. _ Edited by A.Narayana Murthy.
  • Line work must be clean, thin and uniform

Geometrical Transformations

  • skim through pages 67 to 72
  • acquire awareness Affine transformation, Root mean square (RMS) error