Instructor: Dr. M. M. Yagoub

Introduction

List the main types of errors in GIS ?

• Digitization errors (Figure 9.5 page 235): overshoot and undershoot (illustrate by figures)

• Entity error - (position error), primarily associated with vector model (missing entities, incorrectly placed entities, disordered entities)

• Attribute error ( occurs in both vector and raster models, typing errors, misspelling, etc. )

Why be concerned about error?

• GIS usually involves comparisons of many sets of data (coverages, themes). If errors exist in one or all of the data layers, the solution to the GIS problem generated from them may itself be erroneous

Define what is meant by Sliver polygon errors, illustrate?

• Sliver polygon errors Commonly result from incorrect practice of double digitizing

• Can also result from overlay or merging operations which join coverages from different sources

• Can be removed manually or by dissolving polygons less than a certain area and/comparing intended number of polys with actual number

Define Topology?

• Topology is a procedure for explicitly defining spatial relationships connecting adjacent features (e.g., arcs, nodes, polygons, and points).

• Different types of spatial relationships are expressed as lists of features e.g.

• An area is defined by the arcs comprising its border

• An arc is defined by set of points (X,Y)

List the main Topological Concepts, illustrate by figures?

• The three major topological concepts are:

• Connectivity: Arcs connected to each other at nodes

• Contiguity/Adjacency: Arcs have direction and left and right sides

• Area Definition:: Arcs connected to surround an area define a polygon (area)

List the advantages of Topology?

· Check for digitization errors (overshoot, undershoot, unclosed polygon, missing labels, too many labels)

· Store data more efficiently (eliminate data redundancy-normalization)

· Make spatial analysis more faster

What is meant by Coordinate Transformation?

· Conversion of tablet-digitizer coordinates to real world (map) coordinates

List the necessary elements that must be included in a map?

1. Title 2. Content (graphic + attribute) 3. Scale (e.g. 1:100) 4. Legend (symbols) 5. Direction (North Arrow) 6. Name of organization and date of production 7. Frame

Define Continuous and Discrete data?

• Continuous data: objects which have no definite boundary, generally no "empty" space and assumed to have three dimensions X,Y and Z e.g. elevation, temperature, and rainfall. The data is represented as surface in GIS

• Discrete data: objects which occupy a specific location in space at a given point in time e.g. road, river, and lot.and represented as point, line, or area feature in a GIS

Discuss the types of spatial pattern?

• Uniform - regular or evenly spaced points

• Clustered - objects are in close spatial proximity

• Random - no particular pattern (neither uniform nor clustered)

Spatial correlation - the spatial relationship between two variables (positively, negatively). It allows us to quantify spatial patterns

List two advantages of sampling?

1. Time minimization

Cost reduction

List the types of sampling strategies, illustrate by figures?

(see Figure 5.1, page 101)

• Random - all entities have equal probability of being selected

• Systematic (regular) - selection of entities based upon some systematic design e.g., every 10th tree in a transect, soil temperature collected every 100 feet

• Stratified - dividing the population into spatial subsets or thematic subsets before sampling e.g., X number of samples are to be taken from each of 4 plots

• Cluster

• Transect

• Contour

Define Interpolation and Extrapolation?

1. Interpolation: Used to predict missing values when we have values bounding, or on both sides of, the gap (surface fitting models)

– Linear

– Nonlinear (weighted distance)

2. Extrapolation: used when there are values on one side of the missing data, but none on the other side (sampling point data as estimates of areas rather than surfaces e.g sampling number of trees in small areas)