No map is perfect. Even the most accurate maps created by a GIS have some deficiencies. These deficiencies occur due to
“Errors” that may have taken place at different stages of GIS implementation. These errors reduce the accuracy of the map
generated. However by use of well defined and controlled procedures these errors can be avoided.
There are two types of errors in GIS:
I. Source Errors:
They are the errors that are present in “Source Data” that is given to the GIS. They occur before the actual implementation
a. Instrumental inaccuracies:
● Satellite/ air photo/ GPS/ surveying (spatial).
● Inaccuracies in attribute measuring instruments.
b. Human Processing:
● Misinterpretation (e.g. photos), spatial and attribute.
● Effects of scale change and generalization.
● Effects of classification (nominal / ordinal / interval).
c. Actual Changes:
● Gradual ‘natural’ changes: river courses, glacier recession.
● Catastrophic change: fires, floods, landslides.
● Seasonal and daily changes: lake/sea/ river levels.
● Man-made: urban development, new roads.
● Attribute change: forest growth (height etc.), discontinued trail / roads, road surfacing.
II. Processing Errors:
They are the errors that occur during the processing of the data i.e. during the implementation of GIS.
● Digitizing: human error, the width of a line, spikes, knots, also entering attribute data.
● Dangling nodes (connected to only one arc): permissible in arc themes (river headwaters etc.).
● Pseudo-nodes (connected to one or two arcs) – permissible in island arcs, and where attributes change,
e.g. road becomes paved from dirt or vice versa.
● Projection input error.
● Interpolation of point data into lines and surfaces.
● Overlay of layers, digitized separately, e.g. soils and vegetation.
● The compounding effects of processing and analysis of multiple. layers: for example, if two layers each have
correctness of 90%, the accuracy of the resulting overlay is around 81%.
● Density of observations.
● Inappropriate or inadequate inputs for models.
● Scale changes – detail and scale bars.
● Color palettes: intended colors don’t match from screen to Printer.