User:Mav/Introduction to GIS notes by maveric149/2002-02-26 Lecture
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- Generally, this the most accurate data is found here - slightly out of date data can be obtained for a minimal fee.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
Metadata
- 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
Scanning
- 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