b. Errors in Topological Analysis

Logic errors may cause incorrect manipulation of data and topological analyses (Star and
Estes 1990). One must recognize that data is not uniform and is subject to variation.
Overlaying multiple layers of maps can result in problems such as Slivers, Overshoots, and
Dangles. Variation in accuracy between different map layers may be obscured during
processing leading to the creation of "virtual data which may be difficult to detect from real
data" (Sample 1994).

c. Classification and Generalization Problems

For the human mind to comprehend vast amounts of data, it must be classified, and in some
cases generalized, to be understandable. According to Burrough (1986, pp. 137) about seven
divisions of data is ideal and may be retained in human short term memory. Defining class
intervals is another problem area. For instance, defining a cause of death in males between
18-25 years old would probably be significantly different in a class interval of 18-40 years
old. Data is most accurately displayed and manipulated in small multiples. Defining a
reasonable multiple and asking the question "compared to what" is critical (Tufte 1990, pp.
67-79). Classification and generalization of attributes used in GIS are subject to interpolation
error and may introduce irregularities in the data that is hard to detect.

d. Digitizing and Geocoding Errors

Processing errors occur during other phases of data manipulation such as digitizing and
geocoding, overlay and boundary intersections, and errors from rasterizing a vector map.
Physiological errors of the operator by involuntary muscle contractions may result in spikes,
switchbacks, polygonal knots, and loops. Errors associated with damaged source maps,
operator error while digitizing, and bias can be checked by comparing original maps with
digitized versions. Other errors are more elusive.

b) ( Explain any 5, 5x2= 10)

There are five types of data entry systems commonly used in a GIS:

‎input of‏ لا ‎manual digitizing பி scanning‏ لا ‎keyboard entry ப coordinate geometry‏ لا
‎existing digital files‏

‎Keyboard entry: involves manually entering the data at a computer terminal. Attribute data are
commonly input by keyboard whereas spatial data are rarely input this way.

‎Keyboard entry may also be used during manual digitizing to enter the attribute information.
However this is usually more efficiently handled as a separate operation.

‎Roads files versus the census file -- roads file will use codes for the various road types while the
census file uses exact numbers for things like total population, age range, etc.