An overview of editing and data compilation

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 GIS data loses its value if it becomes outdated.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.

This workflow is described in more detail in the next section.

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 - http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Task_examples

Editor Toolbar
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An overview of editing and data compilation