data storage & editing

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