Sept 16 18

1
Sept 16 - 18

• Data entry (digitizing, scanning)
• Editing geodata
• Quality control and error checking
• Tiles
• Edgematching
• Georeferencing and transformations

2
Editing geodata
Once you have completed initial data entry, you
will still need to clean up errors, build higher
level structures, and generally make the data
useable if you are using a simple data structure,
such as a shapefile or grid, this simply means
verifying that data situated in their correct
places if you are higher level data structures,
such as arc/info coverages, you need to verify
that your data are topologically correct, so that
your lower-level objects (lines and points) can
be contstructed into higher-level objects
3
Shapefile structure

• Remember, a shapefile is a simple format for
  geodata
• Shapefiles contain locational information, but
  lacks topology
• Shapefiles allow for arcs to cross
• Shapefiles take up considerably less space in a
  computer, and are more portable across operating
  systems
• Shapefiles have no topological information
  attached

4
More on shapefile structure
The only feature that really needs to be accurate
in shapefiles is location

• four shapefiles
• Country outline
• Cities
• Baringo district outline
• Road network
• the only relevant attribute is that they be in
  the correct location
• they can be used for simple area and distance
  calculations, but not for analysis that require
  connectivity or other topological information

5
More on shapefile structure
Shapefiles lack toplogical information!!!
6
Digitizing from an Air Photo

• Example from last lectuire
• Only relevant attribute is that the boundary is
  in the correct location
• This is correct

7
Digitizing from an Air Photo

• Example from last lecture
• Only relevant attribute is that the boundary is
  in the correct location
• This is not

8
Arc/INFO coverages

• In Arc/INFO, all spatial information has
  topological information attached to it
• Containment, connectivity, adjacency
• to correctly derive this, all coverages are
  subject to planar enforcement
• All intersections of arcs must be joined by nodes
• All polygons must be closed

9
Consider this coverage open for editing
10
Consider this coverage open for editing
11
Consider this coverage open for editing
To determine which arcs make up a polygon, the
computer must locate all arcs which share a
starting or ending node
12
Consider this coverage open for editing
To determine which arcs make up a polygon, the
computer must locate all arcs which share a
starting or ending node
13
Consider this coverage open for editing
It must then find all arcs that enclose an area
14
Consider this coverage open for editing
It must then find all arcs that enclose an area
15
Consider this coverage open for editing
It must then find all arcs that enclose an area
16
Consider this coverage open for editing
It must then find all arcs that enclose an area
17
Consider this coverage open for editing
It must then find all arcs that enclose an area
18
Consider this coverage open for editing
It must then find all arcs that enclose an area
19
Consider this coverage open for editing
It must then find all arcs that enclose an area
All arcs are selected!
These arcs are now known to form the boundary of
a polygon
20
Ideally (for some uses), a node should be the
intersection of three or more arcs
Those that do are called "true nodes" those that
don't are called pseudo nodes
21
Ideally (for some uses), a node should be the
intersection of three or more arcs
Those that do are called "true nodes" those that
don't are called pseudo nodes
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24
This arc did not quite close in on itself, so the
computer has no way of knowing that the (not
quite) enclosed area is a polygon
This is called a dangling node
25
Types of dangling nodes

• Undershoot
• Error, because it does not close up a polygon

26
Types of dangling nodes

• Overshoot
• Error because it does not close a polygon

27
Sliver polygons

• When two arcs are added next to each other, you
  may have some "unintended" polygons added in

28
Sliver polygons

• When two arcs are added next to each other, you
  may have some "unintended" polygons added in