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Data Storage & Editing. GEOG370 Instructor: Christine Erlien. Overview. Storing GIS databases – tiling Types of error that can occur Entity errors Attribute errors Projection and error Edge matching Conflation. Data Storage & Editing Subsystem. Importance - PowerPoint PPT Presentation
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Data Storage & Editing
GEOG370
Instructor: Christine Erlien
Overview
Storing GIS databases – tiling Types of error that can occur
– Entity errors– Attribute errors
Projection and error Edge matching Conflation
Data Storage & Editing Subsystem
Importance– Tools to store & maintain data– Tools to perform QA/QC on data
pinpointing errors
Need to be aware of possible errors to know what to look for in databases you are working with
Data Storage & Editing Subsystem
Types of errors:– Entity error: Error in position
– Attribute error: Incorrect attribution
– Entity-attribute agreement error: Codes associated with wrong entities
Data Storage & Editing
Before beginning a spatial analysis or mapping project check data
Error detection and correction may include:– Compare data with input document– Check topology of spatial objects– Check attributes of spatial objects– Check for missing spatial objects
GIS Database Storage
Raster Data
– Attribute values for grid cells – Associated tables
Editing– Concerned with verification correct
positions of grid cells• Examine column & row position, attribute code
GIS Database Storage
Vector Data
– Individual tables within a single database– Different databases, linked by pointers
Editing– Entities
• Missing objects, incorrectly placed objects, incomplete links
– Attributes• Typos, incorrect codes, attributes incorrectly associated
with a field
– Entities & attributes together• Codes associated with incorrect entities
Vector Data Storage & EditingTiling—storing database in subsections Reduces computational overhead, speeds up
analysis by decreasing data volume Greater control of editing process
From Fundamentals of Geographic Information Systems, Demers (2005)
The importance of editing the GIS database
Sources for error:– Hardware (digitizing, scanning)– People
• Digitizing• Attribute inputs
Detecting & Editing Errors: Vector
Identifying entity errors Build topology Look at database statistics
Check that: All entities that should have been entered are
present No extra entities have been digitized Entities are in the right place & of correct
shape/size Topology is correct Polygons have only a single label point Entities are within the tic mark boundary
Pseudo nodes: False nodes; where a line connects with itself or where two lines intersect along a parallel path rather than crossing.
Entity Errors: Vector
From Fundamentals of Geographic Information Systems, Demers (2005)
Dangling node (dangle): Node connected to a single line entity (doesn’t indicate intersection)
Produced by:– Failure to close a polygon– Failure to connect the node to appropriate object
(undershoot)– Going beyond the entity the node should be
connected (overshoot)
Entity Errors: Vector
From Fundamentals of Geographic Information Systems, Demers (2005)
Entity Errors: Vector
Undershoot Overshoot
Entity errors: Vector
Polygon digitizing Polygon requires point inside that will
act as a locator for a label
Possible errors:– Missing labels– Too many labels– Generally caused by failure to keep track
of the digitizing process
Entity errors: Vector
From Fundamentals of Geographic Information Systems, Demers (2005)
Entity errors: Vector
Sliver polygons: Small polygons produced by digitizing adjacent lines between polygons > once– Occur when digitizing software uses a vector model that
treats each polygon as a separate entity• This type of software becoming less common
– Fix: Compare # of polygons from input map to digital coverage; zoom in to suspect areas
http://www.ncgia.ucsb.edu/cctp/units/unit28/28.html
Entity errors: Vector Weird polygons: Polygons with missing
nodes– Cause: Point digitized in wrong place or wrong
order– Prevention: Map preparation or digitizing method– Fix: Moving lines to correct locations
http://www-users.aston.ac.uk/~elgyj/data_structures.htm
After making changes:
– Rebuild topology• Changes modified some spatial relationships topology
needs to be updated
– Save!!
Incorrect attributes– Prevention: Keep track of attributes as typing in– Fix: Correct them! Save!
Entity & Attribute Errors: Vector
Attribute Errors: Raster
Raster: Missing attributes: Missing entire (or
portion of) row or column
Incorrect or misplaced attributes
Attributes errors occurring along area margins cells may be reassigned to neighboring polygon
From Fundamentals of Geographic Information Systems, Demers (2005)
Dealing with Projection Changes
Coordinate conversion as editing– Cartesian (digitizer) real-world
Projection produced using transformations:– Scale– Rotation– Translation
Some error: Mathematical process of projection & computer rounding– Look at software’s measure of error if high, there
may be problems with digitizing
Edge Matching
Vector Linking adjacent maps (usually the same
theme) to permit analysis of larger study area Potential difficulties:
– Same projection, but digitized separately mismatches
– Different projection or same projection, different datum
– Fix: Link line & polygon entities that should be connected
Edge Matching
Raster Remote sensing products
– Horizontally adjacent scenes collected at different times latitudinal coordinates may differ slightly
• Skew between images• Correctable by shifting grid until matches
Conflation & Rubber Sheeting
Conflation: Process of integrating map data from multiple sources– Generally requires rubber sheeting
Rubber sheeting/Warping– In a reference map, features for which coordinates
known are identified (control points)– Goal: Additional layers made to conform to
reference layer using control points
From An Introduction to Geographical Information Systems, Heywood et al. (2002)
Wrapping up: You should know
What tiling is & its purpose 3 basic types of error & how to edit them
– entity, attribute, entity-attribute
Types of entity error – All entities that should have been entered are present– No extra entities have been digitized– Entities are in the right place & of correct shape/size– Topology is correct – Polygons have only a single label point– Entities are within the tic mark boundary
Types of raster & vector attribute error– How they occur– How detected & corrected
How projection introduces error The purpose of edge matching The purpose & process of edge
matching
Wrapping up: You should know