Spatial data management

Question 1

what is spatial data management?

Answer 1

a group of operations that follow on and is associated with data capturing activities and is used to prepare spatial data for further use.
Refers to the cleaning and fixing of errors in spatial data.

Question 2

What procedures are included in spatial editing?

Answer 2

 Cleaning of captured data by detecting errors and correcting them.
 Generalizing through:
* Coordinate thinning and smoothing,
* Aggregating,
* Classifying,
* Recoding and
* Resampling.

Question 3

What are spatial errors?

Answer 3

can be topological or non-topological errors.

Question 4

What are topological errors?

Answer 4

 Logistical inconsistencies between spatial features
 Poorly or ill defined spatial relationships (i.e. nodes/lines/polygons)
 Errors that violate topological relationships:
1. Connectivity, or the linking of points or polygons to each other.
2. Adjacency or the sharing of a common boundary of two regions
or polygons.
3. Containment or the inclusion of points/polys in areas
4. Contiguity or the direction that arcs have

Question 5

What are non-topological errors?

Answer 5

 Geometric inaccuracies like missing polygons & distorted lines
 Need not be related to spatial relationships
* Variety of basic editing operations that can modify simple
features and create new features from existing features
* Some basic operations are similar to that of topological editing
i.e. Snapping tolerances
* Non-topological editing = spatial editing, BUT no topology gets
defined in the editing process.

Question 6

What are examples of topological errors?

Answer 6

• Undershoot - lines that don’t meet
• Overshoot – Lines that overshoot connecting node
• Polygons that don’t close
• Polygons that overlap

Question 7

What is a sliver polygon?

Answer 7

an artifact of digitizing - consequence
pseudo polygon - that doesn’t actually exist.

Question 8

How to eliminate slivers?

Answer 8

1. Poly option: deletes the longest arc and label point of the
   selected polygon
2. Line option: an arc will be merged with its longest neighbor
   with which it shares a pseudo node

Question 9

What ways yo fix non-topological errors includes?

Answer 9

1. editing existing features
2. creating new features
3. edge matching
4. generalizations.

Question 10

Editing existing features

Answer 10

– Extend/trim lines
– Delete/move features
– Reshape features
– Split lines and polygons

Question 11

non-topological editing creates new features using?

Answer 11

1. Buffer: Create boundaries around a feature at an equal
   distance in all directions, for example zones of different noise
   intensity around an airport
2. Union: Combine features from different layers into one
3. Intersect: Create a new feature from the intersection of
   overlapped features in different layers.

Question 12

How can create New Features by Splitting large data sets into smaller
subsets?

Answer 12

1. Clipping portions out
2. Sub-setting by feature type or some logical selection criteria

Question 13

creating new features by?

Answer 13

Union
Interesect
Clip
Split
Merge

Question 14

Edge matching

Answer 14

Edge-matching is the process to determine which edges (lines) should be linked among candidates. For some cases, one edge will join with only other one and for some other cases, more than two edges will be linked together.

Question 15

generalizations

Answer 15

Generalizing features through: coordinate thinning, coordinate smoothing, aggregating, classifying, recoding and resampling.

Simplify:
– Line
– Polygon

Smooth:
– Line
– Polygon

Question 16

What are geometric transformations?

Answer 16

convert the newly digitized map/data into a projected
coordinate system.
Map projections and Geometric transformations are similar in
concept.
- a map projection converts data sets from 3D format
(Earths sphere) into 2D planar coordinates (Cartesian graticule)
- A geometric transformation converts data sets from 2D digitizer
units/Pixels into 2D planar coordinates (Cartesian graticule)

Question 17

How can geometric transformations be classified in different ways:
Local vs global and relative vs absolute.

Answer 17

1. Local vs. global operations
   * Local operations are relative and spatially constrained, also
   called piecewise and may not affect all features equally
   * Global operations apply some systematic procedure to all
   features
2. Relative and absolute transformations
   * Relative operations are usually local and
   * Absolute are global but not necessarily so

Question 18

What is the definition of geometric transformation?

Answer 18

is the process of using a set of control
points & transformation equations to register a digitized map, satellite
image or aerial photo into projected coordinates.

Question 19

Explain map-to-map

Answer 19

• Newly digitized map (based on digitizer units) converted into projected coordinates.

scanned image to another image.
poly-topographic map to a polygon.

Question 20

explain image-to-map

Answer 20

• Applies to RS. Changes coordinates of rows and columns into projected coordinates.
• Also require resampling to complete transformation. Resampling =
  fills each pixel with a value derived from the original image.
• Misplacement of control points can make transformation result unacceptable!

Question 20

What are geometric transformations performedto?

Answer 20

• (1) improve spatial reliability and to;
• (2) Make geographical data sets compatible with other data sets

Examples of Geometric Transformations:
a) Registering
b) Rubbersheeting
c) Re-projecting - not NB
d) Scaling
e) Translating

Question 21

What is registration?

Answer 21

local and relative operation - can be applied to different section of data
– To spatially register one geographical data set to fit another
– rubber sheeting - Features are moved differentially, to spatially register one geographical data set to fit another, when dataset is distorted (use snapping tool).

Question 22

What is rectification?
not NB

Answer 22

global and absolute procedure
– To transform a geographical data set to some other coordinate system
by applying different mathematical equations
– Often used to transform a digitized map from digitizer coordinate units to the coordinate system of the map that was digitized
– Also called georeferencing

Question 23

What is projection?

Answer 23

global and absolute procedure
– To geometrically transform locations to new positions based on the mathematical properties of the equations used
- no transformations being locally restrained can be applied everywhere.

Question 24

What is rubbersheeting?

Answer 24

• To spatially register one geographical data set to fit another
• Use local adjustments to stretch features
• Features are moved differentially and therefore it is also called rubber sheeting
• Use when one dataset is distorted
• Features are moved differentially, to spatially register one geographical data set to fit another, when dataset is distorted (use snapping tool)

Question 25

Geometric transformations

Answer 25

Equiarea -preserve shape and sizes.
Similarity - preserves shape, not size
Affine - preserves parallel lines
Projective -

Question 26

Resampling

Answer 26

Image to map transformations
Each pixel of a new image is filled with a value or a derived value from the oringal image.

Types of methods:
1. Nearest Neighbor
fills each pixel of the new image with the nearest pixel value from the original image
2. Bilinear interpolation
uses the average of the four nearest pixel values from three linear interpolations
3. Cubic convolution
uses the average of the 16 nearest pixel values from five cubic polynomial interpolations.

Question 27

What is RMSE?
(root mean square error)

Answer 27

measures deviations between coordinate values on a map and coordinate values from an independent source of higher accuracy for identical points.
Can determine the quality of geometric transformation.
If a RMS error is within the acceptable range, we usually assume that the transformation of the entire map is also acceptable.
Measures the displacement between actual and estimated locations of control points.
The RMSE is an accuracy measurement.

Higher accuracy data sources include:
* GPS
* Other digital/hardcopy map data
* Survey data