Читать книгу GIS Research Methods - Steven J. Steinberg - Страница 66

The logical data model adds specific processing steps to the conceptual model developed in the previous step, thus specifying the analytical procedures necessary to complete the analysis. That is, we want to add a workflow to the conceptual figure. This step can be represented graphically by creating a flowchart that specifies each step in specific detail. One unique characteristic about the logical model is that it does not include software-specific processing steps, only the logic behind them.

For example, in our prenatal clinic model, we know we need to extract specified socioeconomic data related to the prospective client population within our particular study area (city). The data source we have identified for this example is the US Census; however, because the census data contain much more information than we really need, we want to pare that down. We want to narrow the geographic extent and, via a database query, extract only those attributes we require. Graphically, this portion of the flowchart might look similar to figure 2.8.

Figure 2.8 A portion of a logical data model showing the steps necessary to reduce the full US Census dataset to the appropriate spatial and attribute components necessary for our study. The logical model first indicates a clip, which is a spatial operation to cut out only the required geographic area, much like a cookie cutter. The resulting dataset is then queried to obtain only those records or attributes needed in the research analysis.

In this example, we specify a logical sequence of steps but do not concern ourselves with how these steps are achieved in the particular software. It is important to have a sense of the possible processing options. Once we have developed a logical flow for the entire analysis, we reach our next evaluation point: determining the feasibility of accomplishing the necessary processing steps using available software tools (figure 2.9). Tools may vary based on the particular software licenses available for data collection, aggregation, and analysis as well as those that may be imposed by outside forces (e.g., field equipment used, versions of software).

Just as data may be brought into the GIS analysis from a variety of software environments, they can also be analyzed and processed in a variety of environments. You may determine that certain steps are better accomplished in a specific database program, a statistical software package, or a variety of other software options. In fact, it is not at all uncommon to incorporate multiple software programs and discipline-specific models into an analysis. For example, aspects of a project in ArcGIS may include data tables produced in Microsoft Excel or imported data from field equipment such as Global Positioning System data from Trimble Pathfinder Office. Many software packages commonly used in a GIS project are easily integrated with ArcGIS, either directly or using ArcGIS tools or other software.

Figure 2.9 An example of a data clip as illustrated in ArcGIS documentation. The dataset on the left covers an area larger than needed. The study boundary (center) is used as a “cookie cutter” to clip the data down to the study region. Esri Resources: Clip (Analysis).

The idea behind thinking through your logic, independent of the software to be used, is to avoid one of the most common pitfalls in GIS analysis: allowing the software, or your knowledge of particular tools, to dictate how you analyze your data. After developing your logical data model, you might have a good basis for selecting the most appropriate GIS software and add-ons (potentially including some new extensions or add-ons) for your analysis. This is akin to every problem looking like a nail when you are only familiar with a hammer. If, in fact, logic dictates using a screw, then obtain and learn to use a screwdriver.

Of course, you can’t completely change software tools for each new analysis. Once your organization has committed to a particular program, you need to take advantage of what it offers, perhaps adding individual extensions or additional tools as needed. Most software, such as ArcGIS, includes thousands of tools and options, more than the typical user can keep track of. So, even if you are not sure how to implement your logical model when you develop it, there is usually room to explore options you may not have considered to determine the best approach to analyzing your data following the logic developed at this step of the abstraction process.

AN ASIDE ON CHOOSING THE RIGHT TOOLS FOR YOUR ANALYSIS

Selecting the right tools for your analysis is essential to conducting research in GIS. It is important to avoid tunnel vision when it comes to the software you incorporate into your analysis. The value of the logical data model is that you think through what you want to accomplish without being distracted by the question of how you will accomplish this from a software perspective. Once you have worked out the logic model, or what you wish to accomplish in your analysis, you can turn to how you can best accomplish it with the physical model.

Selecting the physical tools for the job may require you to go beyond the software tools you already know. Although GIS software packages, such as ArcGIS, provide a relatively complete, out-of-the-box solution, it is not unusual for additional software tools to be required to carry out your analysis.

ArcGIS is compatible with optional software components, called extensions and add-ons, that enhance its capabilities for a particular type of analysis (table 2.1). Extensions and add-ons come in three primary varieties: Esri-developed add-ons, third-party add-ons, and user-contributed add-ons. Although these products can sometimes be expensive, they can pay for themselves in time and effort saved on routine or repetitive analysis tasks. A substantial number of freely available extensions and add-ons are also available. Finally, a wide array of stand-alone software tools (both commercial and free) may be used in conjunction with ArcGIS.

Table 2.1 Extensions and add-ons available from Esri

Extension/add-on	Key benefits
Analysis
ArcGIS 3D Analyst	Analyze your data in a realistic perspective.
ArcGIS Geostatistical Analyst	Use advanced statistical tools to investigate your data.
ArcGIS Network Analyst	Perform sophisticated routing, closest facility, and service area analysis.
ArcGIS Schematics	Represent and understand your networks to shorten decision cycles.
ArcGIS Spatial Analyst	Derive answers from your data using advanced spatial analysis.
ArcGIS Tracking Analyst	Reveal and analyze time-based patterns and trends in your data.
Business Analyst Online Reports	Directly access demographic reports and data from Business Analyst Online (BAO) for trade areas and sites created in the desktop.
Productivity
ArcGIS Data Interoperability	Eliminate barriers to data use and distribution.
ArcGIS Data Reviewer	Automate, simplify, and improve data quality control management.
ArcGIS Publisher	Freely share your maps and data with a wide range of users.
ArcGIS Workflow Manager	Better manage GIS tasks and resources.
ArcScan for ArcGIS (included with ArcInfo and ArcEditor 9.1 and higher)	Increase efficiency and speed up raster-to-vector data conversion time.
Maplex for ArcGIS (included with ArcInfo 9.1 and higher)	Create maps that communicate more clearly with automatically positioned text and labels.
Solution-based
ArcGIS Defense Solutions (includes ArcGIS Military Analyst, Grid Manager, and MOLE)	Create workflows, processes, and symbology to support defense and intelligence planning.
Esri Aeronautical Solution	Use the full power of GIS to efficiently manage aeronautical information.
Esri Defense Mapping	Efficiently manage defense specification-compliant products.
Esri Nautical Solution	A GIS-based platform for nautical data and chart production.
Esri Production Mapping	Standardize and optimize your GIS production.
No-cost add-ons
ArcGIS Editor for OpenStreetMap (download)	Contribute to the OpenStreetMap project by adding, editing, and deleting data within the familiar ArcGIS 10 editing environment.
ArcSketch (download)	Quickly create features in ArcGIS with easy-to-use sketch tools.
Districting for ArcGIS (download)	Create defined groupings of geographic data, such as census tracts, ZIP Codes, and precincts, by creating a districting plan.
Free Geoportal add-ons (download)	Catalog geospatial resources within an enterprise and provide quick access to those resources regardless of location or type.
Geodatabase Toolset (GDBT) (download)	Manage your scalable geodatabases with diagnostic performance tools.
OLAP for ArcGIS (download)	Create, view, use, and manage connections to OLAP databases in ArcGIS for Desktop.
US National Grid Tools for ArcGIS (download)	Support disaster relief and search and rescue with a coordinate system that can be standardized across agencies.
WMC Client (download)	Open Web Map Context (WMC) files directly in ArcMap.

Courtesy of Esri.

You can also develop or add custom scripts, analytical models, or other tools to supplement the built-in capabilities of ArcGIS. These custom components may be available through online user forums for the specific GIS package or by searching on the Internet. Many of these components are just as good as commercial products, and they are inexpensive or free; however, they may lack formal technical support.

If you are working with others, using a common software platform and version and data standards can save headaches when it comes to data compatibility and analysis. In recent years, geospatial data formats have become much more consistent under the leadership of the Open Geospatial Consortium, which boasts more than four hundred members, including geospatial software companies, government agencies, universities, and other organizations from around the world (http://www.opengeospatial.org).

Open-source GIS

Several open-source GIS software tools are available. Open-source software is free to download from a variety of sites on the Internet, and many of the software programs have supportive user communities happy to help out via e-mail lists. In most cases, you need to compile the program to work with your computer, a process that may require additional open-source components that you download independently. However, open-source software can be an excellent option if you are comfortable working with software with its configurations, and sometimes these tools provide analytical capabilities not readily available with other software.