Redding Holiday Inn and Convention Center
Wednesday, October 21, 1998
Room 1
8:30 a.m.--10:00 a.m.

WOW! A WHOLE NEW WAY OF LOOKING AT YOUR DATA

The first ArcView session will present material aimed at people who are just starting out with ArcView or who are interested and would like to see what a desktop Geographic Information System (GIS) can do for them.

The session will begin by addressing some fundamental concepts which are essential for providing a firm foundation for GIS. This will be followed by a "live" ArcView presentation during which the participants will learn rudimentary skills and how to generate effective maps using their existing permit and use data together with a few readily available base data layers.

There is a lot of material to cover in the space of 90 minutes, and so the following background material has been prepared to give participants some additional information.

Desktop GIS represents the real world on a computer in the same way maps represent the world on paper. However, desktop GIS has power and flexibility that paper maps lack because GIS provides an almost unlimited amount of information about what can be seen on a map. GIS stores all the information about map features in a GIS database and links features on the map to the information about them.

Maps: Maps are graphic representations of the cultural and physical environment. Two subclasses of maps exist: general purpose maps--such as USGS 7.5' Quad sheets and thematic maps. Thematic maps show the spatial distribution of some geographical phenomenon--such as pesticide use, endangered species habitat, etc. Since the real world is infinitely more detailed than a map can be, we say that maps give a generalized view of the real world. Natural and man-made objects, such as rivers and roads, can all be represented on maps and are referred to as map features.

Features: Because features on maps are organized according to relative position or location, maps are good for showing the relationships between feature locations. These relationships are called spatial relationships. To represent real-world objects, maps use three basic shapes, points, lines, and polygons (areas). Points represent objects that have discrete locations and, depending on the scale of the map, are considered too small to be depicted as polygons. Lines represent objects that have length but, again depending on mapscale, are too narrow to be depicted as polygons. Polygons, or areas, represent objects too large to be depicted as points or lines. Since shapes alone do not give enough information about a feature, maps use graphic symbols and text labels to help identify features further.

Attributes: The information that GIS stores about map features is referred to as attributes. Attributes include a wealth of information about a feature, including name, length or area, ownership, use, etc., to list but a few. GIS formats attributes in rows and columns, and stores them as tables. Each column stores a different attribute, and each row relates to a single feature. The link between map features and their attributes is the basic principle behind how a GIS works, and is the source of its power. Because the link between features and attributes is dynamic, changing an attribute in the table automatically results in a change in the map.

Themes: Desktop GIS links sets of features and their attributes and manages them together in units called themes. A theme contains a set of related features such as roads, parcels, or waterways, which have a set of common attributes. For example commodity field sites would be grouped into a theme because they all represent parcels on which there is the potential for growing a commodity.

GIS Database: All themes for a geographic area taken together make up a GIS database. The themes in the database can be used together to analyze multiple situations and solve multiple problems. New themes can be added, and old ones deleted. Themes can be separated to create more themes, or combined to consolidate information.

Spatial Data: Perhaps the most important concept to consider when using GIS is the quality of the spatial and tabular data that is used. Provided the user understands the limitations of the data being used, and conveys this information in the map interpretation, then most data is of value. All data used in a GIS should have metadata, which is usually a text document identifying the data source, the scale at which the data was created, the projection/coordinate system units, a listing of the attributes, etc.

Referencing Spatial Data--Understanding Map Projections: The locations of map features are referenced to actual locations of objects that they represent in the real world. The positions of objects on the earth's surface are measured in degrees of latitude and longitude, and are referred to as geographic coordinates. Geographic coordinates may be expressed in units of degrees, minutes, and seconds (e.g.--120 deg., 30 min., and 0 sec.) or as decimal degrees (e.g.--120.5000). On a flat map, the locations of map features are measured in a two-dimensional planar coordinate system. These coordinates describe the distance from an origin (0,0) along the X and Y axes. Because the earth is round and maps are flat, getting the information from the earth's surface to a flat map requires the use of a mathematical formula called a map projection. This process of "flattening" can result in distortions of both distance and area, depending on the type of mathematical projection used. In California, which covers more area in a N-S direction versus an E-W direction, the Albers Equal Area projection allows the whole state to be included in just one zone, compared to UTM which divides California into two zones along the -120th parallel, and the state plane coordinate system which divides the state into 6 or 7 zones in a N-S direction. For this reason, the Teale Data Center and DPR, maintain all their spatial data in the Albers Equal Area projection. However, many counties use the state plane coordinate system because the zones of this system are oriented along county boundaries and this projection offers minimal distortion over these smaller county-sized areas. Desktop GIS makes it easy to display spatial data from a variety of sources at the same time, provided that all the data is in the same map projection.

Detail--Determining What is Important: Another important consideration when using spatial data is the amount of detail that is required. This will depend on the type of information needed. Field sites represented as points don't give information about their shape, and it is not possible to measure their dimensions or locate features, such as waterways, contained within them. On the other hand, point locations probably best represent apiaries, since it is not important to know the shape of the bee hives.

Scale--Understanding Why Different Datasets Don't Match Up: Some projects require a higher degree of locational accuracy than others. For example, if a feature representing a road or parcel boundary is mapped to within 40 feet of its real world location, it may not be accurate enough for an engineering application or legal parcel assessment, but it may be perfectly adequate for a CAC wishing to identify the location of a commodity or pesticide use. It is important to note that while spatial data of all map scales (e.g. 1:1,000,000 to 1:1200) can be displayed in the same view by a GIS, their relative positions with respect to one another will vary greatly due to their accuracy. For spatial data generated at 1:24,000 (e.g. a USGS 7.5' Quad sheet) 1 inch represents 2,000 feet, whereas data at a scale of 1:250,000, 1 inch represents approximately 4 miles. While data at these scales (1:250,000) may be useful, particularly if no other spatial data is available, it is important to bear in mind the relative accuracy of the mapped features compared to their actual locations on the ground. This is especially important to consider when the information is used to make judgements concerning distances of less than 1,000 feet. In many instances counties which have well established GIS programs in their planning or public works departments have often developed data layers at a scale of 1:4800 or better where 1 inch represents 400 feet (a ground accuracy of +/- 10 feet). This spatial data provides GIS users with greater accuracy, providing them with the potential for making better locational decisions.

Where to Find Data?: Data sources abound, and is becoming less expensive and more available. DPR has limited data resources which can be made available to CACs for DPR related projects. Free data is also available from the USGS, Census Bureau, and some state agencies. If your county has a well established GIS program, the CAC may be able to share data with other departments. Vendors collect, package, and sell data for a wide range of applications, and some county-sized spatial data sets are available at a reasonable cost. However, there is little or no data currently available specifically related to agriculture in California, and in many cases CACs will have to consider the investment of creating their own information, particularly a crop parcel layer.

How to Create Good Spatial Data: A desktop GIS allows users to create spatial data by drawing shapes over the top of existing maps or images. Several CACs, including Fresno and Kern, have almost completed this task for their crop parcel layers using a combination of pre-existing assessor parcel data, street centerline data, and aerial imagery as the base data layers. While existing parcel lines and street centerlines offer CACs a tremendous head start, imagery also supplies many important details such as field boundaries, water features, etc., that may be absent from many data layers.

The "live" ArcView demonstration will explore some of ArcView's many features, show participants how to load data, perform simple analyses, and generate output. The demonstration will include the following steps:

How ArcView is Organized: ArcView stores maps, charts, and tables in a project. A project is a file for organizing all the information, and is a way of storing documents so that it may be rebuilt each time a project is opened.

Documents: There are five document types, Views, Tables, Charts, Layouts, and Scripts. During this presentation we will focus on views, tables and layouts.

	Views--	display sets of geographic data (themes) as interactive maps
	Tables--	display tabular data
	Layouts--	display views, tables, and charts together for display or printing purposes (Layouts can be printed, plotted, and pasted into other applications.)

How to Create a View: Spatial data can be added from a number of data types. Themes will be added from ArcView shapefiles, Arc/Info coverages, textfiles containing geographic coordinates (such as GPS data), and imagery. The view's Graphical User Interface (GUI) will be examined, and many of the menu choices, buttons and tools demonstrated. The view's Table of Contents (TOC) will be discussed, and the themes will be manipulated to show the effects of changing the TOC's draw order, a theme's activity and visibility status, and a theme's legend classifications.

How to Use Tabular Data: Each theme of geographic data in a view has a table that stores attributes describing features it contains. Theme attribute tables will be examined together with the menu choices, buttons and tools of the table document's GUI. The dynamic link between the tabular data and the geographic data will be demonstrated using spatial selection tools and simple tabular queries.

How to Use External Tabular Data: In many cases information in addition to data available in the attribute tables may be related to thematic data from external tables. ArcView provides users with ways to match data from related tables based on the values of a common field found in both tables, e.g. township/range/section. External data tables will be added to the project from a variety of sources including a database document, a textfile, and a Microsoft Access database. It will be demonstrated that there are two ways in which to relate data in ArcView, either by a join or a link.

Join--A join should be used if the relationship between a theme's attribute table and an external table is a one-to-one or many-to-one relationship. For example, there might be an external table (or lookup table) containing information about crops, and a theme table (the crop parcel attribute table) with a crop-type field. The tables can be joined together based on crop type so that the detailed information about the crops can be displayed in the view.

Link--A link establishes a one-to-many relationship between a theme's attribute table and an external table. One record in the theme's attribute table (e.g. the crop parcel attribute table) is related to one or more records in the external table (e.g. pesticide use data). Unlike joining tables, linking tables simply defines a relationship between two tables, rather than appending the fields of the external table to those in the destination (the PLS themes' attribute table). When tables are linked, neither table is changed--they are just linked to one another.

Selecting Data Using External Data: Data selection using tabular and spatial queries will be demonstrated with both joined and linked datasets.

Creating New Datasets: New datasets can be created either by converting existing features into shapes, or by drawing new shapes. The first ArcView session will focus on the creation of datasets from existing themes. New datasets will be created from Arc/Info coverages, textfiles, and selected features in existing themes. A demonstration of editing techniques and methods for creating new datasets by drawing new shapes will be addressed in the second ArcView session.

Creating Map Layouts: ArcView allows users to present information in views, tables or charts, together with scalebars, north arrows, titles, etc., at the same time on either a screen or piece of paper in one document called a layout. Using some of the documents created during the demonstration, a layout will be created and some of the layout's frame features examined.

References:

Getting to Know ArcView GIS. 1996. ESRI. Published by GeoInformation Internation.
ISBN 1 86242 019 X.

Using ArcView GIS. 1996. ESRI. ArcView GIS Software Documentation.