data storage & editing

27
Data Storage & Editing GEOG370 Instructor: Christine Erlien

Upload: bracha

Post on 20-Jan-2016

59 views

Category:

Documents


0 download

DESCRIPTION

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

TRANSCRIPT

Page 1: Data Storage & Editing

Data Storage & Editing

GEOG370

Instructor: Christine Erlien

Page 2: Data Storage & Editing

Overview

Storing GIS databases – tiling Types of error that can occur

– Entity errors– Attribute errors

Projection and error Edge matching Conflation

Page 3: Data Storage & Editing

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

Page 4: Data Storage & Editing

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

Page 5: Data Storage & Editing

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

Page 6: Data Storage & Editing

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

Page 7: Data Storage & Editing

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

Page 8: Data Storage & Editing

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)

Page 9: Data Storage & Editing

The importance of editing the GIS database

Sources for error:– Hardware (digitizing, scanning)– People

• Digitizing• Attribute inputs

Page 10: Data Storage & Editing

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

Page 11: Data Storage & Editing

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)

Page 12: Data Storage & Editing

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

Page 13: Data Storage & Editing

From Fundamentals of Geographic Information Systems, Demers (2005)

Entity Errors: Vector

Undershoot Overshoot

Page 14: Data Storage & Editing

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

Page 15: Data Storage & Editing

Entity errors: Vector

From Fundamentals of Geographic Information Systems, Demers (2005)

Page 16: Data Storage & Editing

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

Page 17: Data Storage & Editing

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

Page 18: Data Storage & Editing

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

Page 19: Data Storage & Editing

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

Page 20: Data Storage & Editing

From Fundamentals of Geographic Information Systems, Demers (2005)

Page 21: Data Storage & Editing

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

Page 22: Data Storage & Editing

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

Page 23: Data Storage & Editing

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

Page 24: Data Storage & Editing

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

Page 25: Data Storage & Editing

From An Introduction to Geographical Information Systems, Heywood et al. (2002)

Page 26: Data Storage & Editing

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

Page 27: Data Storage & Editing

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