Chap 7

Front 1

Location errors refer to

Back 1

the geometric inaccuracies of digitized features

Front 2

Location errors can be examined by

Back 2

referring to the data source for digitizing

Front 3

Causes of Digitizing Errors

Back 3

Human Errors

Scanning and Tracing Errors:Collapsed lines Misshapen lines Extra lines Duplicate lines

Conversion Errors of Digitized maps to into real-world coordinates :Use a set of control points Errors will exhibit regular pattern Must re-digitized control points and rerun geometric transformation.

Front 4

new method for creating new spatialdata

Back 4

Using GPS and Satellite imagery data,

Front 5

When using GPS or remote sensing, the map scale does not contribute to thespatial data accuracy, only

Back 5

the resolution of the source data is relevant

Front 6

Spatial Data Accuracy Standards, three phases

Back 6

U.S. National Map Accuracy Standard

2. Accuracy standards for large-scale maps proposed by the American Society forPhotogrammetry and Remote Sensing in 1990

3. National Standard for Spatial Data Accuracy established by the FederalGeographic Data Committee in 1998

Front 7

Standards for Horizontal Accuracy

Back 7

No more than 10% of well-defined map points shall be more than• 1/30 inch at scales larger than 1:20,000 (threshold of 40 feet on the ground)• 1/50 inch at scales of 1:20,000 or smaller (threshold of 167 feet on the ground)

Front 8

ASPRS (1990): Accuracy standards for large map scales

Back 8

Horizontal accuracy defined in terms of RMS• 1:20,000 scale maps RMS threshold of 16.7 feet (5.09m)• 1:2400 scale maps, RMS threshold of 2 feet (0.61m)

Front 9

Spatial Data Accuracy measures

Back 9

how close the recorded location of a spatialfeature is to its ground location

Front 10

Data Precision measures

Back 10

how exactly the location is recorded

Front 11

Distance measurements:

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• Decimal digits• Rounded to the nearest foot or meter

Front 12

Numbers can be stored as:

Back 12

Integers

• Floating point• Single precision, with 7 significant digits• Double precision, with 15 significant digits

Front 13

Topological errors

Back 13

violate the topological relationships either required by a datamodel (e.g., the coverage) or defined by the user

Front 14

How many topological rules

Back 14

31

Front 15

Topological errors with Line features include:

Back 15

• undershoot, (gap)

• overshoot, (over extended line)

• dangling node, results from undershoot or overshoot

• pseudo node, appears along a continuous line and divides the line unnecessarily into separatelines.

• direction error, (exp streams)

• label error. If a polygon has two or more label point, it is an error

Front 16

Topological errors with Polygons:

Back 16

• Unclosed Polygons• Gaps between polygons• Overlapping polygons

Front 17

An overshoot (left) and an undershoot (right). Both types oferrors result in dangling nodes

Back 17

Front 18

(a) An unclosed polygon,(b) a gap between two polygons, and(c) overlapped polygons

Back 18

Front 19

Pseudo nodes, shown by the diamond symbol, are nodesthat are not located at line intersections.

Back 19

Front 20

The from-node and to-node of an arc determine the arc’s direction

Back 20

Front 21

Multiple labels can be caused by unclosed polygons

Back 21

Front 22

Topological errors between layers include

Back 22

boundaries not coincident: between two polygons, or larger polygons notsharing boundaries with smaller polygons.

• lines not connected at end points: between tow layers, example highwaysand roads are expected to connect perfectly across administrativeboundaries.

• overlapping line features: example rail lines on highways, and line featuresnot covered by another set of line features (e.g. bus routes not covered bystreets)

Front 23

A powerful tool in ArcGIS for topological editing is

Back 23

cluster processing

The processing uses a cluster tolerance, also called XY tolerance, to snap vertices (i.e., pointsthat make up a line) if they fall within a square area specified by the tolerance

Front 24

The default cluster tolerance is

Back 24

.001 m

A cluster tolerance should not be set too large because a large cluster tolerance can unintentionallyalter the shapes of lines and polygons

Front 25

Vertices to be snapped can be in the same layer or between layers T/F

Back 25

T

Front 26

A map topology is

Back 26

a temporary set of topological relationships between the parts of features that aresupposed to be coincident.Therefore, editing using map topology can ensure, for example, that a land-use layer and a soil layershare the same study area boundary.

Front 27

To edit using map topology

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1- Create a map topology2- Specify the participating feature classes3- Define a cluster tolerance4- Use editing tools in the GIS package

Front 28

Editing with topology rules involves three basic steps

Back 28

1- Create a new topology by defining the participating feature classes. In afeature dataset,The rank of each feature classThe topology rule(s)The cluster tolerance

2- Validation of topology: this step evaluates the topology rule and createserrors indicating those features that have violated the topology rule.Topology rules will be saved to a topology layer

3- Using a topology layer for fixing errors and accepting errors andexceptions. (e.g. acceptable dangling nodes)

Front 29

Nontopological EditingBasic Editing Operations on existing features

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Front 30

Nontopological operations can create new features by:

Back 30

Merging features: group selected line or polygon features into one feature (fig 7.19)

Buffering features: to create a buffer around a line or polygon feature at a specified distance.

Union features: to combine features from different layers into one layer

Intersecting features: to create a new feature from the intersection of overlapped features indifferent layers.

Other editing operations include:EdgematchingLine simplificationLine smoothing

Front 31

Edgematching

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matches lines along the edge of a layer to lines of an adjacent layerso that the lines are continuous across the border between the two layers

Example: Edgematching is required for creating a regional highway layer that ismade of several state highway layers digitized and edited separately

Front 32

Shortest path analysis can only be used

Back 32

unless all errors between these differenthighway layers are removed

Front 33

Edgematching involves

Back 33

a source layer and a target layer

Front 34

Line simplification

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refers to the process of simplifying or generalizing a lineby removing some of its points

Front 35

Line smoothing

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refers to the process of reshaping lines by using somemathematical functions such as splines

Line smoothing is more important for data display

Front 36

Douglas-Peucker algorithm

Back 36

used for line simplification, it works line byline and with a specific tolerance.