Editing and data compilation
An overview of editing and data compilation |
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Release 9.3
Last modified May 14, 2009 |
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Introduction to editing and data compilation
Compiling geographic information is the most costly and time-consuming part of developing a GIS. The geographic information that is created is a valuable but time-sensitive asset, so care and precision must be taken in its collection and maintenance. GIS data loses its value if it becomes outdated. The database must be updated to keep it current. Just as ownership changes when a parcel is sold, the geometry of the parcel feature itself may change if it is split for subdivisions. Even natural features change over time: the course of a river may change through meandering or by stream capture.
To minimize the cost of data development and necessary updates, it is important that the tools and workflows for data compilation be as streamlined and productive as possible. ArcGIS has a complete data compilation toolset. ArcMap is the application for creating and editing geographic data. ArcMap contains tools that help you construct features quickly and easily while maintaining the integrity of your data. For organizations that need multiple users to simultaneously edit a shared geodatabase, ArcMap and ArcSDE provide the tools necessary to manage versions and resolve potential conflicts.
ArcGIS allows you to create and edit several kinds of data. In general terms, the feature geometry you can edit includes points, lines, polygons, text (annotations and dimensions), multipatches, multipoints, shared edges and nodes from a topology or geometric network, and tables. Some types of data, such as CAD drawings or coverages, can be viewed but not edited inside ArcMap and must be edited in the data's native application. For example, although you can view coverages in ArcMap, you can only edit them in ArcInfo Workstation. You can import these display-only formats into geodatabase feature classes or shapefiles and edit them in ArcMap.
Whether you're using ArcView, ArcEditor, or ArcInfo, you use the same editing tools in ArcMap to work on your geographic data. Certain editing operations may require additional licensing such as ArcEditor, ArcInfo, or an ArcGIS extension. For example, ArcView allows you to display but not create or edit some geodatabase capabilities such as topologies, dimensions, geometric networks, and data in an ArcSDE geodatabase.
The process of compiling and editing data in ArcGIS can be divided into two categories of operations: collecting data and integrating data. Data collection involves entering new data into your GIS, while data integration involves transforming data and maintaining its integrity.
Methods of collecting data
Digitizing data
Digitizing—the process of converting features into a digital format—is one way to create new data. There are several ways to digitize new features. These include digitizing on screen or "heads up" over an image, digitizing a hard copy of a map on a digitizing board, or using automated digitization.
Interactive, or heads-up digitization, is one of the most common methods. In this method, you display an aerial photograph, satellite image, or orthophotograph on screen as a basemap and trace features, such as roads, buildings, or parcels, from it.
In hard-copy digitizing, you use a digitizing table connected to a computer that converts positions on the table surface into digital x,y coordinates as you trace them with a handheld puck (a pen or mouselike device).
Automatic digitization is another method of digitizing features. The ArcScan for ArcGIS extension enables you to perform automatic or interactive raster-to-vector data conversion with high precision, with little or no operator intervention during the data capture stage.
Collecting data in the field
Some GIS data is directly captured in the field using a Global Positioning System (GPS) device. GPS units calculate their position using signals from satellites (and sometimes base stations). They vary in capability and accuracy, so be sure to use a GPS that is as accurate as the data with which it will be used. GPS units can be connected to handheld computers, laptops, or Tablet PCs to record data in the field.
Handheld computing devices allow you to run ArcPad software, which provides database access, mapping, GIS, and GPS integration to users out in the field by handheld and mobile devices. You can extract data from a geodatabase to use in the field with ArcPad, then you can update your geodatabase with the changes you made in ArcPad. If you use a notebook computer or a Tablet PC in the field, you can bring the actual geodatabase and its full capabilities with you.
Learn more about integrating data from a GPS
Learn more about integrating data from ArcPad
Learn more about using a Tablet PC with ArcGIS
Integrating survey data
Survey or coordinate geometry (COGO) measurements can also be entered into a GIS. Within ArcMap, you can create lines or polygon edges using a set of survey measurements collected in the field. In addition, you can use the ArcGIS Survey Analyst extension to store and work with survey measurements collected from field notes, survey equipment, and data collectors.
Workflow for creating and editing features
Before you create or edit features in ArcMap, you need to have an existing feature class to edit. If you don't have one, you can create a new geodatabase feature class or a shapefile in ArcCatalog.
The Editor toolbar contains the various commands you will need to edit geographic features in your database. You must add the Editor toolbar to ArcMap before you can begin editing.
Once you have added the data you want to edit to ArcMap, you'll follow a basic workflow.
- Choose the workspace and data frame you want to edit.
- Start an edit session (start editing).
- Choose which layer within your workspace you want to be the target of your actions (the target layer).
- Choose whether you want to create new features or edit existing ones.
- Set up additional properties or options such as turning on snapping, setting which layers are selectable, and specifying input units.
- Choose a tool. The Editor toolbar contains the most frequently used simple-feature editing tools. Additional toolbars contain tools for editing topologies, geometric networks, routes, and annotation features.
- Add or edit attributes of the feature.
- Save edits and stop editing.
Most of the time you will edit data in data view, since it shows only the data in your map and hides the layout elements. You can edit in layout view, although editing is typically easier and more accurate in data view. However, editing in layout view is useful when you want to make additions and modifications to your map in the context of your map layout. For example, suppose in data view you have added some text that identifies an ocean, but in layout view, you see that the text is too close to the edge of the data frame. To correct this, you can simply put the data frame into focus and move the piece of text while you're still in layout view, rather than switching between views.
Choosing the workspace and starting an edit session
Editing applies to a single workspace in a single ArcMap data frame, where a workspace is a geodatabase or a folder of shapefiles. If you have more than one data frame in your map, you can only edit the layers in one data frame at a time—even if all data is in the same workspace. ArcMap can display data stored in different coordinate systems in the coordinate system of a data frame. Although you can edit data in different coordinate systems, it is generally best if all the data you plan to edit together has the same coordinate system.
Editing occurs in an edit session. During an edit session, you can create or modify vector features or tabular attribute information. When you want to edit, you need to start an edit session, which you end when you're done. Edits are temporary until you choose to save and apply them permanently to your data. You can also quit an edit session without saving your changes. Just saving a map document does not save the edits to the features—you need to specifically save the edits in your edit session.
When you save edits, you write them to the data source, or a database. When you are working with data in a database, making edits and saving them are transactions against the database. Versions allow multiple users in a multiuser geodatabase to edit the same data without applying feature locks or duplicating data. When users are ready to apply their edits, they will merge their changes through a process of reconciling edits, resolving conflicts, and posting their changes to the parent version of a database.
Learn more about database transactions
Learn more about versions
Setting the target layer
Once you have chosen the workspace that will be edited, many editing operations require that you specify the layer that will be the target for your edits. For example, when you are creating new features or modifying existing features, you need to set the target layer so ArcMap knows the layer you want to update. You must set the target layer whenever you create new features—whether you're creating them with the Sketch tool, by copying and pasting, or by buffering another feature, as some examples. You set the target layer with the drop-down list on the Editor toolbar rather than the table of contents. Highlighting a layer in the table of contents has no effect in the ArcMap editing environment.
The Target layer drop-down list (shown below) contains the names of all the layers in the datasets with which you're working. Subtypes are also listed, if applicable. For instance, if you set the target layer to Parcels > Non-Residential, any features you create will be part of the Non-Residential subtype of the Parcels layer.
When you choose a certain layer as the target, certain geodatabase properties are carried over to the ArcMap editing environment. For example, when you create a new feature in a layer based on a feature class with geodatabase domains specified, these attributes will automatically be present in the new feature. Similarly, when you choose an annotation layer as the target, the display properties of color, size, font, and so on, you've specified for the geodatabase feature class will automatically be set up for you.
Some operations do not require a target layer, since they obtain their settings from other properties in ArcMap. For example, you do not need to specify a target layer when you simply want to select a feature, then move, rotate, or edit its vertices. In cases of selection, for example, the ArcMap editing tools adhere to the ArcMap settings such as selection options and which layers are selectable.
Choosing an edit task
The editing operations you can use on features are called edit tasks. One of the most common tasks is to create new features. The tasks are categorized based on their general function such as to create data, modify data, and work with topology. You specify the task from the drop-down list on the Editor toolbar. You can customize the task list by adding and removing tasks, based on the needs of your project. In addition, you can use programming to develop your own editing tasks.
Choosing a tool and creating an edit sketch
In ArcMap, you use an edit sketch to create or modify the shape of features. An edit sketch is a temporary representation of a feature that allows you to edit its geometry.
A sketch is composed of all the vertices and segments of the feature. Vertices are the points at which the sketch changes direction such as corners; segments are the lines that connect the vertices.
When you want to create new features or draw a sketch, you'll most commonly use the Sketch tool and the tools on the Editor toolbar tool palette. With those tools, for example, you can create lines, arcs, and tangent curves; vertices at intersections or midpoints; vertices based on distances and directions from other features; or new segments by tracing along existing ones.
The Sketch tool has an accompanying shortcut (context) menu that helps you place vertices and segments more accurately. For example, you can add a vertex at a specific x,y location, draw a segment at an exact length and direction, or make a segment parallel or perpendicular to another segment. You can also set up any additional options for editing such as turning on snapping.
When you create the vertices in a sketch (typically, by clicking with your mouse), the segments between vertices are added automatically. Once you're satisfied with the shape of the sketch, you need to finish the sketch to complete the feature's geometry and actually create the feature. There are several ways you can finish a sketch including double-clicking with your mouse, choosing the command from a shortcut menu, or using a keyboard shortcut (F2).
The left graphic below shows the polygon feature being constructed from an edit sketch. Once all the desired vertices are added, the sketch is finished and it becomes a feature. You can double-click a feature with the Edit tool to modify the sketch (thus, changing the shape of the polygon).
To create a new polygon with the Sketch tool, for example, at least three vertices are required. To create a new line, at least two vertices—the start and endpoints—are required to finish the sketch and, therefore, create the feature. A sketch of a line records the direction it was digitized, which is important if you need to trim or extend the line or perform an operation on it that uses measurements originating from the start or endpoint. You can flip the direction of a line so the last vertex of the sketch becomes the first. Vertices are marked in green, with the last vertex added marked in red.
Sometimes, you need to create a feature that has more than one physical part but only references one set of attributes in the database. These are called multipoint for point or multipart for line and polygon features. A system of oil wells could be represented by a multipoint feature, which can only be created in a feature class with multipoint geometry. (The multipoint geometry type is set when you create a new feature class.) Multipoint layers are created as sketches. Similarly, a group of islands could be represented as a multipart polygon feature. To create a multipoint or multipart feature, you need to finish each part first, then finish the whole sketch once you have created the individual parts.
Examples of edit tasks and sketches used together
You can use the tasks in conjunction with a sketch you create to edit your geographic data in ArcMap.
Here are some examples:
- The Create New Feature task uses a sketch you create to make the new feature.
- The Select Features Using a Line task uses a sketch you create to select features; the features the line intersects are selected.
- The Cut Polygon Features task uses a line sketch you draw to cut a polygon.
View illustrations of how these tasks are used
Editing existing features with sketches
In addition to constructing new features, sketches can be used to modify existing features. For example, to reshape a polygon, you can construct a sketch of the new edge, or you can use a sketch to divide one polygon into two.
You can insert or delete vertices in a sketch to modify a feature's shape. You can also trim and extend lines and split lines and polygons by drawing a sketch. When you use a sketch to modify an existing feature, you complete the update by finishing the sketch.
Adding attribute information
Attributes are descriptions of a geographic feature in a GIS, usually stored as a row in a table. For example, attributes of a river might include its name, length, and average depth. You can enter new attribute values when you create features, and you can edit existing values. When you create a new feature, it starts with only the default attribute values. You input attributes after you create a feature.
You add and modify the attributes of features within an edit session. You can edit attribute values manually by typing in new values or by copying and pasting existing values. Some features are designed with subtypes, default values, and attribute domains. These can make it easier to edit values and can help prevent data entry errors. After you modify attributes that have range domains, you can validate your edits to check that the new values fall within the valid range.
If there are relationships between feature classes and tables in your geodatabase, you can find the objects related to a particular object and edit them. For example, if you have a table of landowners that is related to a parcel feature class, you can select a parcel feature, use the relationship class to find the owner of that parcel, then edit some of the attributes in the owner table. Some relationship classes have rules that control how features can be related. After you edit related geodatabase features or tables that have relationship rules, you can validate your edits to check that the related objects still conform to the relationship rules. You can also establish a new relationship between objects or break existing relationships.
Learn more about editing with subtypes, default values, and attribute domains
Learn more about editing relationships and related objects
Learn more about creating relationship classes
Methods of editing attributes
There are two main ways of adding or updating attributes in ArcMap: the Attributes dialog box and the table window.
The Attributes dialog box displays attributes of selected features and shows you the raw data and fields as they are stored in your geodatabase. The left side of the dialog box shows the name of the layer to which the selected feature or features belong, while the right side shows the attribute values.
You can also open a table window to see all the tabular information about a layer or table. The table window displays field aliases, which are often more user-friendly names for your fields, and can hide the fields you don't want to see. To add or change a value in the table window, simply click a cell and type the new value. The table window also allows you to use the Field Calculator, which helps you update multiple values in a particular field and allows you to use advanced Visual Basic for Applications (VBA) statements when editing field values. Although you don't have to be in an edit session to calculate field values, an edit session is recommended because it allows you to undo your edits.
Updating data using SQL
You can use Structured Query Language (SQL) outside ArcGIS to perform bulk attribute updates on your data. However, you need to have a good understanding of your data model when using this approach to ensure that the chance of data corruption is minimized and any attributes you update do not affect other objects in the database through relationships or other behavior.
Learn more about updating data with SQL
Creating new text features
You can store map text as annotation in a geodatabase. Annotation provides flexibility in the appearance and placement of your text because you can select individual pieces of text and edit them. Dimension features are a form of annotation designed to express distance measurements. Geodatabase annotation and dimension features are created and edited inside an edit session.
You can create and edit annotation with the specialized annotation editing tools. You'll follow a similar process as with editing other features: choose the layer and the method you want to use to create the annotation and use the Sketch tool to create the feature.
Some annotation feature classes stand alone in the geodatabase. These are known as standard annotation. Standard annotation is not formally associated with features in the geodatabase. For example, you might have a piece of standard annotation that represents a mountain range, ocean, or administrative boundary; the annotation simply marks the general area on the map.
Another kind of annotation, known as feature-linked annotation, is associated with the feature it is describing through a relationship class. The text reflects the value of a field or fields from the feature to which it's linked. You might use feature-linked annotation to identify features such as parcels, streets, rivers, roads, or cities. With feature-linked annotation, as you create new parcels or street features, for example, using the editing tools in ArcMap, annotation will be created automatically.
You can create dimension features with different methods and modify where they are placed and how they look. Dimension features are stored in dimension feature classes and can be placed and edited using the tools on the Dimensioning toolbar.
Enhancing productivity while editing
Many editing operations can be made more efficient and accurate using some of the functions in the editing environment.
The snapping environment
Snapping is one of the easiest ways to more accurately position new vertices and segments as well as features as you move them. Snapping can help establish exact locations in relation to other features. When snapping is turned on, your mouse pointer will jump, or snap to, edges and vertices when your pointer is near them. This means the pointer is within the snapping tolerance. Snapping can help you with many editing operations, for example, creating polygons that do not overlap or have gaps between them or placing a point exactly along an existing line.
Learn more about the snapping environment
Shortcut menus, keyboard shortcuts, and distance units abbreviations
The shortcut (context) menus for many tools, especially the Sketch tool, provide commands that help you place vertices and segments more easily and accurately. You open a shortcut menu by right-clicking the map with a particular tool.
Many editing functions have keyboard shortcuts associated with them including those on the shortcut menus. If you learn some of the most common shortcuts, you can minimize the use of your mouse and speed up your edits. For example, with the Sketch tool active, you can press the F6 key to quickly enter the coordinate location of the point or vertex you want to place. You can also switch to a different editing tool or quickly change to a navigation tool to zoom or pan the map by pressing different keys.
Sometimes you may need to enter lengths or other measurements in units different from your map units. In many dialog boxes throughout the editing environment that require you to enter a distance value, you can specify values in different units of measure by simply typing a unit abbreviation after the number. For example, if your map units are feet, by default, ArcMap will assume any distance values you enter are in feet. However, you can simply add "m" after your input value so ArcMap knows the value you entered is actually in meters.
Caching data for better performance
When you are editing data in a geodatabase, especially an ArcSDE geodatabase, you may want to turn on the map cache, which allows temporary storage of geodatabase features from a given map extent in the desktop computer's memory. Building a map cache can reduce the load on your network and the geodatabase since ArcMap accesses this information from your computer's RAM, reducing the number of queries the client needs to execute on the server.
Learn more about the map cache
Opening additional windows
You can also use additional windows in ArcMap—magnifier, overview, and viewer windows—to get different views of your data, which can help when editing. You can get a closer view of a small area without changing the map extent (magnifier window), see the full extent of your map (overview window), or get an independent view onto the map (viewer window). For example, a magnifier window can be turned on before or during the creation of new features and modification of existing features. You can click the heading of the magnifier window and drag it while holding on to the same sketch or modification already in progress. Viewer windows can be used on computers with dual monitors, so you can open and maximize the viewer window on the second monitor to provide a large-scale view of the area currently being edited. Editing can be performed in both displays, and graphic feedback shown while you edit appears simultaneously on both displays.
Learn more about magnifier, viewer, and overview windows
Integrating data
GIS data often comes from many sources. Inconsistencies between data sources sometimes require you to perform additional work to integrate a new dataset with the rest of your data. Some data is geometrically distorted or rotated with respect to your base data. You may also need to edit boundaries that are shared by multiple features or implement additional methods of maintaining the integrity of your data.
Performing spatial adjustments
Within the editing environment, the spatial adjustment tools provide interactive methods to align and integrate your data. Spatial adjustment supports a variety of adjustment methods and will adjust all editable data sources. It's often used when you've imported data from another source, such as a CAD drawing or a 3D multipatch model. Some of the tasks you can perform include converting data from one coordinate system to another, correcting geometric distortions, aligning features along the edge of one layer to features of an adjoining layer, and copying attributes between layers. Since spatial adjustment operates within an edit session, you can use existing editing functionality, such as snapping, to enhance your adjustments.
In addition, using a magnifier window when you're performing spatial adjustment allows you to get a better view of the origin and destination locations so you can place links more accurately without changing the extent of the map. You can open a magnifier window above the start point of the link, add the link, then move the window to the destination location and add that link without having to zoom and pan the map.
Editing shared geometry
You may find that you need to edit the geometry of multiple features at once. Using the topological association among features, you can move boundaries and vertices shared by multiple features. For example, you can move a border to update two forest polygons or, as seen in the graphic below, move a corner vertex and update several parcel polygons and a few lot boundaries at the same time.
Within the editing environment, you can create a map topology that enables you to edit the shared parts of features. A map topology is a temporary set of topological relationships between coincident parts of simple features on a map. The primary types of geometry that are acted on when editing a map topology are edges, which are line segments that define lines or polygons, and nodes—points at the end of an edge. When you move a node in a topology, all the edges that connect to it are stretched to stay connected to the node. When you move an edge, edge segments stretch to maintain the connection of shared endpoint nodes to their previous location. You can also move a node and a connected edge without stretching the other connected edge by temporarily splitting the topological relationship between the node and the other shared edges.
When creating a map topology, you need to specify which layers will participate in the topology and the cluster tolerance to use to determine which parts of the features are coincident and which edges and nodes in the topology are shared.
Maintaining spatial integrity
Beyond editing shared geometry, topology within a geodatabase allows you to maintain additional spatial integrity by specifying a set of rules to apply to your data, allowing you to find and fix any errors and validate the relationships.
Topology
Topology is a collection of rules that, coupled with a set of editing tools and techniques, enables the geodatabase to more accurately model geometric relationships. ArcGIS implements topology through a set of rules that define how features may share a geographic space and a set of editing tools that work with features that share geometry in an integrated fashion. A topology is stored in a geodatabase as one or more relationships that define how the features in one or more feature classes share geometry. The features participating in a topology are still simple feature classes—rather than modifying the definition of the feature class, a topology serves as a description of how the features can be spatially related.
For example, in the graphic below, the coral-colored squares and lines indicate errors in topology, which is one way of ensuring spatial integrity. These may be locations where there is a line that does not connect to another line (known as a dangle), a polygon that does not contain a point within its boundary, or some other violation of a rule imposed on the geodatabase.
ArcGIS provides a set of editing tools that help find and fix topology rule violations. Violations of topology rules are initially marked as errors in the topology, but where appropriate, you can mark them as exceptions. Within the editing environment, you can use the tools on the Topology toolbar to locate errors, fix errors interactively or automatically, validate edits, and create new features from the geometry of existing features. Dirty areas track the places where topology rules may have been violated during editing. Depending on your workflow, you can validate topology rules and clean up dirty areas after each edit session or on a schedule.
Learn more about topology
Geometric networks
Geometric networks also store various mechanisms and behaviors that maintain the topological connectivity between their component features. Geometric networks consist of edge network features and junction network features. An example of an edge feature is a water main, and a junction feature might be a valve. Edges must be connected to other edges through junctions.
Geometric network connectivity rules constrain the type of network features that may be connected to one another and the number of features of any particular type that can be connected to features of another type. By establishing these rules along with other rules, such as attribute domains, you can maintain the integrity of the network data in the database. At any time, you can selectively validate features in the database and generate reports as to which features in the network are invalid—that is, are violating one of the connectivity or other rules.
Edit operations that involve adding, deleting, and moving geometric network features can all affect the connectivity of a geometric network. Connectivity between network features is maintained on the fly as you create, delete, and modify network features. For example, when a network edge or junction is moved, the network features to which it is connected respond by stretching and adjusting themselves to maintain connectivity. You can use the geometric network editing tools to modify, rebuild, and verify connectivity.
Learn more about geometric networks
Maintaining attribute integrity
ArcGIS provides tools to help you maintain the quality of your attribute data, primarily through the use of attribute domains.
Using attribute domains
Attribute domains are rules that describe the permissible values of a field type and are used to constrain the values allowed in any particular attribute for a table, feature class, or subtype. Each feature class or table can have a set of attribute domains that apply to different attributes. A given attribute domain can be shared by various feature classes and tables in a geodatabase. If a feature class has subtypes, each subtype can have a different domain associated with a given attribute. For example, a water mains feature class and a water laterals feature class can use the same domain for their ground surface type fields.
When you edit a field with a coded value domain, a drop-down list of all the domain values appears. When you choose one of the values from this list, you know that you are assigning the field a valid value.
A range domain specifies a valid range of values for a numeric attribute. For example, you could have three subtypes of water mains: transmission, distribution, and bypass. Each of these could have a different range of valid pressures. Distribution water mains can have a pressure between 50 and 75 psi. For a distribution water main object to be valid, its pressure value must be between 50 and 75 psi. A range domain specifies this range of values.
Result on attribute domains after splitting and merging features
Often when editing data, a single feature is split into two features or two separate features are combined, or merged, into a single feature. For example, in a land base database, a land parcel may be split into two separate land parcels due to rezoning. Similar zoning changes may require two adjacent parcels to be merged into a single parcel.
The behavior of an attribute's values when a feature is split is controlled by its split policy, and when two features are merged, an attribute's value is controlled by its merge policy. Each attribute domain has both a split policy and a merge policy. When a feature is split or merged, ArcGIS looks to these policies to determine what values the resulting feature or features will have for a particular attribute.