GIS.pdf
- 1. The Geodatabase GIS Topics and Applications
- 2. Geodatabase vs Other Formats • Coverages and Shapefiles stored geospatial and attribute data in different locations in different formats – .shp (proprietary binary format) – .dbf (dBase database format) • Geodatabases store both geospatial and attribute data in the same structure
- 3. Benefits and Drawbacks • Benefits – GIS data can now be handled like most other data, and stored in a RDBMS – Greater flexibility and functionality – “Enterprise” level of managing data • Drawbacks – Speed hit – Even more rope to hang yourself with
- 4. ESRI Geodatabases • File Geodatabase – Introduced in 9.2, the File Geodatabase is the latest, greatest file-based format from ESRI • Personal Geodatabase – Introduced in 8.x – Based on Microsoft Access/Jet Engine • ArcSDE – Software (now part of ArcGIS core) that allows RDBMSs to act as GIS data stores.
- 6. File Geodatabase • Latest file-based format • Best modern format for large datasets • Very efficient use of storage space • What you should be using for significant work • Stores data on disk in several files within a directory named geodatabase.gdb
- 7. Personal Geodatabase • Based on Microsoft Access • Great for bringing outside data into ArcGIS • Limited to 2GB • Becomes slow as amount of data increases • Stores data in one file called geodatabase.mdb
- 8. ArcSDE/Enterprise database • Most likely stored on an entirely different machine from the one you’re running ArcGIS on • Same basic functionality as other GDBs • Can store versions of the GIS features, allowing you to see changes over time • Concurrent users (multi-user and replication) • Managed (hopefully) by a DB administrator
- 9. Working with Geodatabases • At a minimum, consider it similar to a subdirectory with shapefiles • Unlike shapefiles, you can enforce extents, storage types, projections, topology rules, connectivity rules, network-specific rules, and so on • This additional functionality is implemented through Feature Datasets
- 10. Feature Datasets • A “folder” within the GDB, it preserves projection and extent information for data within the folder (“feature classes”) • To make it useful, you must set extent and projection information • Put some forethought into it before specifying projection and extent!
- 11. Feature Datasets • After creating a GDB, right click and choose New > Feature Dataset • The dialog boxes will step you through setting the variables for the Feature Dataset
- 12. Importance of Extent • The Geodatabase will only bother with the information within the extent • It will throw an exception if you attempt to put something that doesn’t fit in the box • ArcGIS can preserve the difference between two points down to the molecular level • Setting the extent allows you to control the precision at which ArcGIS handles data • Needlessly too precise, and you’ll have errors that’ll never show up on the screen, but will still impact your data
- 13. Defining New Jersey • Projection: NJ State Plane (feet) • Extent: ? – Should it be tight? – Should it extend outside the boundaries?
- 14. Defining New Jersey • In this case, Arc defaults to a grid of 0.00328 feet • Roughly 4/100ths of an inch • About a hair’s width • 0.2 feet is slightly smaller than 1/4”
- 15. Balancing Precision and Functionality • Your extent should match the scale in which you are working • Leave a little wiggle room • Working in New Jersey? Some of NY, PA, DE should fall into your box. • Greenland fits? Your box is a little too big.
- 16. Additional Functionality • In your Feature Dataset, right click and see what pops up under New > • Topology • Geometric Network • Network Dataset • Etc…
- 17. Geodatabase as a container • Each of these “special” datasets uses the GDB to store data specific to its framework • Topology stores associated attribute tables, rules, and error information • Network stores network edge attributes, turn tables, and driving/routing directions
- 18. Normalization • A normalized database is one that has little redundancy within its tables • Record ID or some other key links to a table with those values • Instead of storing “Modified Agricultural Wetlands” numerous times as text, store it once as text and refer to it using a key (2140)
- 19. Normalization • Work in a normalized environment • Analogs: – Non-normalized: Excel Spreadsheet – Normalized: well made Access DB (lookups) • When distributing for the public, “flatten” the database out to one table per layer • Make it a shapefile
- 20. Geodatabase Environment • Important to work in a GDB whenever possible – Assured extents, projections, etc – Quality control – Greater number of tools at your disposal • Export to other format (.shp) for distribution
- 21. Going Further
- 22. Standard Query Language • SQL is the standardized method of interacting with a database • Even Access allows you to use SQL • Insert (new records into a DBMS) • Update (existing records in DBMS) • Delete (remove records from DBMS) • Where (limits your results)
- 23. Select Statements • Most common SQL query you will encounter • “Select By Attributes” has this as the foundation • Nothing more than “SELECT * FROM gis_layer WHERE…”
- 24. Joins • In ArcGIS or Access, you join two (or more) tables together using a primary key. • If the keys match, the secondary tables are tacked on to the first • Again, geospatial is special, so GIS has another type of join
- 25. Spatial Joins • Relationship not determined by key, but by proximity or connectivity • Contains/Within/Overlaps – One feature falls entirely within another • Touches/Intersects/Crosses – One feature touches another • Equals or Disjoint • List of spatial relationships.
- 26. Relations • Joins work for one-to-one relationships, where one record in a table matches to one (and only one) record in a foreign table. • Often, data requires the use of a one-to-many or many-to-many relationship. • In GIS, joins are strictly 1-to-1. Relations allow the GIS user to access more complicated relationships in the database.
- 27. Transactions • Geodatabase edits are either committed or rolled back • Edits performed in a multi-user environment are integrity checked • Atomic-level editing and revisioning • Needed to prevent a race condition
- 28. Versioning • GIS tracks edits made and maintains a journal of all changes to the database • This record keeping allows for roll backs to any date on record • Keep one set of records while reverting another • Same database methodology as Wikipedia
- 29. Data, data, everywhere • In the Internet age, massive amounts of data are compiled, transmitted and analyzed every second • Understanding the storage and retrieval methods are critical • Difference between drinking and drowning