What is a Geodatabase Topology?
What is Geodatabase Topology?
A geodatabase topology is a set of reference lines that define the spatial relationships between the contiguous rows in a table.
Topologies are used to keep data organized and related and to ensure that data located near one another will be joined together when queried or analyzed.
Geodatabases store geographic features such as point, line, and polygon features. Geodatabase topologies can be applied to tables containing feature geometry data such as points, lines, or polygons for which linear order is important.
Elements of a Geodatabase Topology
- The name of the to-be-created topology.
- In topological processing tasks, the cluster tolerance is employed.
- Geometry feature classes and rules are listed here. All must be arranged into the same feature dataset and in the same coordinate system.
- The rank of the relative accuracy of the coordinates in each feature class. If certain feature classes are more accurate than others, you should give them a higher coordinate rank.
Designing a Geodatabase Topology
A feature can be in only one geometry type, so the topology must contain information about where features should be placed into the topology.
Topologies can either be planned or improvised. A planned topology becomes a schema during database design.
When creating a geodatabase topology, you:
- Create a list of the feature classes that will be shared geometrically.
- Define the spatial representations for each feature class (point, line, or polygon).
- Identify the feature classes that will share geometry and be changed and maintained in tandem. For example, if you change the shape of one of the features, the others will also be modified.
- Combine these feature classes to form a feature dataset.
- Define the topology rules that apply to the items in each unique feature class. Parcels, for example, might be single-part or multipart polygons. Geometry is shared by adjacent lots. Parcels cannot be overlapping.
- Define the topology rules that apply to feature classes. For example, parcel boundary lines must encompass the lines that define parcel polygons.
- Determine the accuracy ranks of each feature class’s coordinates. The most accurate feature classes should be assigned a rank of one, and lower accuracies should be ordered in decreasing order from the highest rank (the second most accurate gets a rank of 2 and so on).
The accuracy rank of many feature classes might be the same. If you can’t see a difference in accuracy, set the rankings of all feature classes to 1. (In other words, so the accuracy is the same).
- Create a test geodatabase with a copy of your features (using a file or a personal geodatabase) to test your suggested topology design. Test your geodatabase prototype.
Add a topology based on your suggested design, validate it, and tweak some of the characteristics, for example. This will provide you with information into behavior and allow you to fine-tune your design.
- Fine-tune and tweak your design until you have a functional implementation.
Map Topology vs Geodatabase Topology
A map topology is a topology that has been created from a map, whereas a geodatabase topology is one that has been created from a bill of materials.
The goal is to attempt to satisfy all requirements, regardless of the modeler’s level of understanding.
Testing, validation, and refinement
A topology should be tested, validated, and refined. Topologies are dynamic and can change. You might realize after the fact that a newly created feature class has an accuracy rank that is less accurate than you anticipated or that a boundary feature class does not fit the parcel you originally intended.
You must test your geodatabase topology for its ability to meet the requirements of your application/schema.
The topology rules, for example, might result in a geodatabase containing the geometry of different accuracies (e.g., the accuracy of a point is less accurate than the accuracy of a line/polygon) or in a single feature class with multiple boundaries.
- You will know if your topology is meeting requirements when:
- You can query a geodatabase containing different coordinate types (e.g., best fit, no fit, and close).
- You can query a projection that matches your geographic coordinate system on the same data.
- There are no overlapping features between rows in the data source and other tables used by your application.
- The count or percentage of each feature type being in the geodatabase matches the count or percentage determined during testing.
- It is easy to define and maintain a geodatabase containing a particular set of features.
- You should always validate your geodatabase topology by testing it against the application requirements.
Why validate a Geodatabase Topology
The purpose of validation is to check that the database is being properly constructed and that the rules followed during construction are valid. It ensures that the results of your analysis and visualization are accurate.
Topology rules may change during implementation or be updated after implementation. It is important to document how you created your topology so you can easily bring it up to date later if needed.
The most efficient way to test a geodatabase topology involves:
- Running the topology and testing for collisions by querying a test dataset against rows in your geodatabase.
- Querying the data source to ensure that geometry types are identical.
- Existing features of your design might not be readily available or accessible, so you might need to create them before determining if they can meet requirements.
- Existing features and ignoring geometry types can lead to incorrect results, so it is best to test the rules against all feature types available.
- It is important to test with a data set that includes all of your features, including the boundaries of your custom features.
- New features, new rules, and old geometry types might not be readily available or accessible, so it is best to create them before determining if they can meet requirements.
- When testing a feature class that is topologically complex (e.g. it has multiple boundaries and geometry types), wait for the query to finish running, since you might receive different results for each combination of geometry type and boundary type.
- Querying using a geographic dataset can help you find problems with topology rules when your geodatabase is being queried from a non-geographic location, such as ArcMap.
- Use data from your test dataset to help you determine which topology rules are correct for your environment, which topology rules are more important, and where the requirements may be ambiguous.
A topology error can occur in a geodatabase if the topology rules for joining features are altered after feature geometry has been added to a data source.
If you alter the position of features or alter the order of features, it could cause error. If your application is dependent on accurate feature coordinates but does not have adequate error checking and correction, it could lead to a topology error.
If the error is not detected, it might not be noticed until runtime, or it may never be noticed.
Geodatabases are dynamic and changing. Topology rules may change during implementation or be updated after implementation. It is important to document how you created your topology and how you will manage it as you add features and alter rules.
What to fix when error occurs
When you encounter a topology error, first try to find the cause of the error by:
- Examine the error message.
- Examine the data source, either directly or indirectly (via a query).
- Examine the rules that you used to create your topology.
- Document what is happening inside your geodatabase when an error occurs so you can limit errors in future implementations of your design.
- Try to determine if the error area is correctable without having to update other parts of your design and/or without re-running the topology entirely.
- Create a new topology if the error area is in correctable but does not impact the results of your analysis and visualization.
How to ignore errors for a testing environment
You might want to allow yourself to create complex geodatabase topologies that contain geometry of different errors in order to test and test again.
You should not allow errors to be set in your geodatabase so that the geodatabase is tested against clean and clear data.
How to fix errors in a topology
This is done by:
- Create a new topology using existing features inside your existing topology.
- Correct any rules that you changed during implementation and re-run the topology.
- Enlarge or shrink boundaries until the error stops causing error or until all geometry types stop being different from each other for each boundary type.
- Change the topology rules to ensure that all boundaries are the same type and to ensure that all geometries are the same type.
- Create a new feature class if no errors now exist in your topology, and re-run the topology.
- If you created some geometry types inside your application, remove them and add a new feature class with only one geometry type in it until you can determine which geometry types caused your error.
- Update your documentation to include the new errors that you found and how they were resolved.
- Test again using an existing dataset to ensure that all errors have been resolved.
Why Geodatabase Topology Is Important
Topology is important in a geodatabase because it describes how you joined all of your feature classes, which attributes are included and where they are located, and which rules are required to form relationships between features.
Topology is important in a project because it describes how certain types of data will be joined as they exist in the system.
Topology is important in analysis because it describes how to put all of the features together to form analysis, such as a grouping, aggregation, or other types of analysis.
Topology is important in a report structure and layout because it describes the spatial relationships between various items.
Advantages of Geodatabase Topology
Geodatabase topology enables a variety of benefits to the organization and users of your geodatabase.
- The ability to use all types of data without having to rebalance the data or re-calculating joins as you place features in your geodatabase.
- The ability to view all features from any location, as well as change any feature attributes at any time, when an analysis is being performed inside a report or application (e.g. changing the view of the dataset to include a new attribute bucket that was created for a requirement).
- The ability to use complex geodatabase topologies with large datasets without the memory being an issue.
- The ability to use features from different datasets in an analysis without having to add them as tables or perform any other data management operations inside or outside of an application.
- The ability to use geography as a topping layer in an analysis.
- The ability to use a specific set of features and no others in an application, even when those features are inside a dataset that has many other features.
The more complex your geodatabase topology is, the more benefits you will be able to observe and enjoy.
Disadvantages of a Geodatabase Topology
Geodatabase topology also has some disadvantages, as well.
- It can take time to learn and implement.
- It can be difficult to determine when it is right to use local or global rules and how or when those rules should be turned on or off, which can lead to confusion or the requirement of an additional layer of mapping inside an application that shows what the global rules are versus what the local rules are.
- It can be difficult to determine which rules or features were created during implementation and which ones were added later.
- It can be difficult for another organization to join datasets into a geodatabase and reuse them.
- It can take time to update an existing geodatabase because it is not known exactly when all of the changes that occurred inside the geodatabase could have affected the topology and when an error could have occurred inside an application.
What is topology?
Topology is the method and rules you used to join your features, which attributes are included and where they are located, and which rules are required to form relationships between features.
What is geodatabase topology?
Geodatabase topology is the rules that are enforced when the geodatabase is queried or updated.
What is a data source?
A data source is the feature class or shapefile that you joined your other feature classes to.
What are topology rules?
Topology rules are the rules that come with the data source and define how each type of geometry will be joined to each other type of geometry.
What are topology errors?
Topology errors are when one or more geometries are joined incorrectly. An error in a dataset can cause an error in a project, analysis, report, etc.
How do you fix a topology error?
If you can detect the type of errors that are causing an issue in your geodatabase, you can correct them.
Why is topology important in a geodatabase?
Topology is important in a geodatabase because it describes how to put all of the features together to form an analysis or report. It also describes how certain types of data will be joined as they exist in the system.
What is wrong with the current geodatabase implementation?
You have made a topology error if you have altered the position of features or if you have altered the order of features and it has changed how your data works. A topology error occurs when a dataset has different errors in it, in which case your application may not be able to function as intended.
How do you search through a geodatabase?
Search inside your geodatabase by searching everything. Any feature, class, or attribute exists. You can also search in all of the attributes of some feature classes so that a very large number of features are included in the search to speed up your query and implementation time.
How does geospatial data get created from a topology?
When you create your topology in ArcMap, you have to create every rule. You can then follow those rules for your future analyses and visualizations.
How does data get created from a topology?
Data can be created from a topology by following the rules that you have established in your geodatabase.
Why is ArcGIS Pro a good option for implementing topology?
ArcGIS Pro allows you to create your topology using all of the data sources and features inside them, as well as alter the design while you are creating it.
Why was ArcGIS Pro created?
ArcGIS Pro was created to allow users to create topologies and manage them inside of ArcMap.
What is topology error in GIS?
Topology error is when one or more geometries are joined incorrectly. An error in a dataset can cause an error in a project, analysis, report, etc.
What is the best way to fix topology errors?
The best way to fix topology errors is to create a new topology using existing features inside your existing topology and have it corrected any rules that you changed during implementation.
Then enlarge or shrink the boundaries until the error stops causing error or until all geometry types stop being different from each other for each boundary type. Change the topology rules to ensure that all boundaries are the same type and to ensure that all geometries are the same type.
How do you build a topology in GIS?
You build a topology by joining your features and adding rules to enforce them. If you have added a new feature, you need to add the same rule that was already there, or in some cases add rules for additional relationships.
How do you create a large topology?
You create a large topology by joining all of the features in your data source. The size of your data source will determine how large your topology is. If you have created a new feature, then you need to add the same rule that was already there, or in some cases add rules for additional relationships.
Why is topology required in GIS?
Topology is required in GIS because without it there may be some errors, and without it you will not be able to query your geodatabase or visualize your data.
What are the uses of topology?
Topology has many uses in GIS. Some of them include:
- Topology is used to connect a feature class to another feature class, allowing you to join data so that it is displayed on the same map as it was created.
This is done by answering two questions: how do you get from one feature class to another and how do you join two features together.
- Topology is used to determine the size of a rectangle, which decides how the geodatabase will be represented on the map.
- Topology is used for the battle over space in your data. It gives users control over how many features can be displayed on a single map and how large those features can be.