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 & Editing Tiling —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 UndershootOvershoot

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

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

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