Resolving Geospatial Data Challenges in QGIS
Learn how to overcome issues in managing spatial data within relational databases using QGIS. Explore the impact of exporting multi-line geometries and discover solutions to describe shapes effectively.
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Presentation Transcript
A QGIS attribute table plugin that describe shapes By Philip Whitten (MGIS student at Penn State) Supervisor James O Brien
A large store of data accumulated from a wide range of sources wide range of sources within a company and used to guide management decisions* Data Warehouse A database structured to recognize relations between stored items of information* relations Relational database Specifies how some object is located in space in relation relation to some reference object+ Spatial Relation *Oxford Living Dictionaries, Oxford University Press +Wikipedia
Problem Many organisations receive and send a broad variety of spatial data sets and store these in relational databases. QGIS is popular for maintaining and analysing the datasets stored in relational databases QGIS is very open and versatile when it comes to geospatial geometry and this versatility can cause problems!
Feature geometry changes when exporting a QGIS Multi-Line memory layer These changes: are without warning have unexpected and unintended consequences create barriers to integration Original (QGIS Memory) Geopackage Shapefile Spatialite PostGIS MS SQL Multi-Line Multi-Line Multi-Line Multi-Line Multi-Line Multi-Line Line Multi-Line Multi-Line Multi-Line Line Line Empty Empty null Empty Empty null null Empty null Empty null null
Solution Make QGIS describe geospatial geometry Relational Datasets Describe Shapes QGIS
Relational Datasets Spatial Datasets - Sets
Relational Datasets Spatial Datasets - Relationships
Relational Datasets Sets The subset ? = ?,? of the universal set ? = ?,?,?,?,? A B a i o u e ??????? = ??????,??????,?????? ?????? ?? ??????? ??? = 1,2,3
Relational Datasets Set Intersection a g ?,?,?,?,? ?,?,? = ? o u i s e ?,?,?,?,? ?,?,? = a l e o f u i y = empty set
???? ? ???? ? = ? ????????????? ????? (2,1) ???? ? ???? ? = ? ????????????? ???? ? ???? ? = ? ????????????? ????? 1,1 ,(3,1) Empty shapes occur whenever the set operation or topology means that no geometry exists Relational Datasets
Empty shapes enable spatial relationships in a single dataset Square Squares (shape) ?????? ???? ?????? ??? ? Left square Right square Point Type Points (shape) ?????????? ?1,?2,?4 ?????????? ?3 Blue crosses Red circles ?????? ??????? Point Type Blue crosses Red circles Blue crosses Square Left square Left square Right square Shape Multi????? ?1,?2 ?????????? ?3 ?????????? ?4 Point Type Left square (shape) Multi????? ?1,?2 ?????????? ?3 Right square (shape) ?????????? ?4 ?????????? Blue crosses Red circles
Many GIS datasets are constrained to a single shape column You can t directly show spatial relationships in a single dataset without multiple shape columns! Trigger point for confusion between a GIS professional and a Database professional Point Type Left square (shape) Multi????? ?1,?2 ?????????? ?3 Right square (shape) ?????????? ?4 ?????????? Blue crosses Red circles
Relational Datasets Set variables Let x be an element of ? = ?,?,?,?,? A a i o Hence, x = ? , x = ? , x = ? , x = ? , or, x = ? Unknown elements of a set can t be shown on a Venn diagram Unknown elements of a set are denoted as null in most databases. u e
Relational Datasets null s in datasets = unknown Road Id Rd1 Rd2 Rd3 Rd4 Road Name Picton null Appin Road Type Road Lane Lane null Road Id Rd1 Rd2 Rd3 Rd4 Road Id Rd1 Rd2 Rd4 Road Name Picton Appin Road Id Rd1 Rd2 Rd3 Road Type Road Lane Lane
nulls are always about Road Id Rd1 Rd2 Rd3 Rd4 Road Name Picton null Appin Road Type Road Lane Lane null Line null ???? ((2,0),(2,2)) ???? ((1,1),(1,3)) ???? ((0,2),(3,2)) Road Id Rd1 Rd2 Rd3 Rd4 Road Name Picton null Appin Road Type Road Lane Lane null Road Id Rd2 Rd3 Rd4 Line ???? ((2,0),(2,2)) ???? ((1,1),(1,3)) ???? ((0,2),(3,2))
Null or empty geometries are not invalid Empty geometries are defined in the OGC GeoPackage Encoding Standard (QGC 12-128r10) The concepts of Empty and null are universal to set theory and to relational databases. Checking validity of empty or null geometries with QGIS will not report any errors.
QGIS The attribute table in QGIS hides the Shape attribute! ?
The QGIS Attribute Table Shows Records That Don t Have a Shape!! QGIS
Problems Identified Problems 1. Null and empty shapes are parsed differently by different QGIS data providers 2. Datasets containing multipart geometries are parsed differently by different QGIS data providers 3. Many GIS professionals don t anticipate or check for null or empty shapes 4. Empty shapes were excluded from the original QGIS and GDAL specifications 5. The QGIS attribute table hides the shape column/s.
Plugin Design Paint by Item Delegate Virtual Field QT Table View
Current icons used in the plugin There are many more geometry types that could be incorporated!
Resources http://plugins.qgis.org/plugins/geomAttribute/ Give my plugin a go! https://github.com/PhilipWhitten/geomAttribute All of the source code and documentation https://philipwhitten.github.io/geomAttribute/html/index.html The API and help files
