Introduction to ArcView GIS Module 3

Developed for the NRCS: by ESRI and National Cartography and Geospatial Center

Copyright Environmental Systems Research Institute, Inc.

INTRODUCTION TO ARCVIEW CONTENTS

Module 1. IntroductionAnatomy of ArcView 3.2ArcView Basic Skills

Theme ManipulationSymbology and Legend Development

Module 2.ArcView Basic Skills

Create a New Theme-Features and TableEdit Existing ThemesCreating and Exporting Layouts

Module 3.Geospatial Data BasicsAcquiring DataData Management

Components of a GISINTRODUCTION

PEOPLE DATA

CaptureStoreManipulateAnalyzeDisplay

UsingSOFTWAREHARDWAREPROCEDURES

Module 3

Objectives - Achieve a level of understanding about Digital Data required to complete Task 6 and Task 7 of the Customer Service Toolkit Users Guide.

INTRODUCTION This course has been modified from a classroom format to a self-study using MS Power Point on CD-ROM and is intended that the student interact simultaneouslywith ArcView 3.2 as each function and feature of the software and data is discussed in the course. An effort is made to provide step-by-step instructions to accomplish course objectives. These instructions are in the yellow insets.

In Power Point, print the presentation in “Notes Pages” format to obtain the text.

Start ArcView 3.2.

In the “Welcome to ArcView window, Click on “Open an existing project”. Browse to the training project (training.apr) file saved at the end of Module 2. There should be a View named Sample Data. Resize and position the ArcView and MS Power point windows on the computer screen as displayed.

What Should YouYou Know About Data Quality and National Map Accuracy Standards?

Geospatial Map Positional Accuracy - map features agreement with features “on the ground”. Data Quality Map Attribute Accuracy - agreement of data content depicted with actual conditions.

National Map Accuracy Standards (NMAS) •Published by the Office of Management and Budget.•Based on results attained with cameras.•Last revised in 1947•Specifies that 90% of well-defined points must fall within a specified tolerance determined by the map scale.

National Standard for Spatial Data Accuracy (NSSDA)•Published by Federal Geographic Data Committee (FGDC)•Developed to evaluate digital geospatial data•Final Phase-FGDC Endorsed 1998•NMAS horizontal accuracy reported according to NSSDA is as follows

Scale CE95(ft) 1 : 7,920 25.3 1 : 12,000 38.4 1 : 24,000 45.6

Example of soil map unit outlines with 45 ft buffer on a DOQ.

Geospatial Data Quality -

Scale In your Arc View session, change the scale ratio of the Sample Data view to 1:20000. This is the scale the soil map was created and is a critical element determining the amount of detail about the soil pattern that can be clearly displayed. Using this scale minimizes the risk of misrepresenting the accuracy of the information.

Enlarge the scale ratio to 1:7920. Though it is a common practice to enlarge a soil map it does not result in a more detailed or accurate map than was created at the 1:20000 scale.

The GIS user must be aware of the scale limitations of data layers when considering acquiring data and when overlaying various data layers to perform GIS analysis.

Detailed information about the data is found in the Metadata file which should accompany all datasets.

What Should YouYou Know About Projections, Datum, and Coordinate Systems?

Projection MethodsPlanar - a flat surface is tangent* to the globe at a single point or lineCylindrical - a cylinder is wrapped around the globe, and is tangent to one arc of the globe (line of latitude or longitude)Conic - a cone contacts the globe at two points or lines of tangency *Maps have no distortion at points/lines tangency

Coordinate SystemsCoordinate Systems are for referencing locations on a globe or map, for example Geographic-latitude/longitude is a spherical coordinate system.

Most maps have planar coordinate reference systems, such as UTM or State PlanePlanar coordinates are based on the Cartesian coordinate concept of x,y as a location east and north of a point of origin.

Origin (0,0)

x axis

y axis

Data usually here

ProjectionProjection is a method to portray the features of a curved surface (the globe) onto a plane (a flat surface like a map).

Datum Datum are used to adjust projections and planar coordinate systems to irregularities in the shape of the Earth (e.g. mountains, trenches).Examples: NAD27 and NAD83 (North American Datum 1927 and 1983) have different coordinates for the same location.

Basic ArcView Projected Data

In your Arc View session dismiss the Sample Data view and create a New View. Enter the View/Properties dialog box and change the name to Projection NAD83. Change the Projection Category to UTM 83 and Projection Type to Zone 14.

Add the dc.sid image to the new View. Then try to add the soil_a_ shapefile to the view. This should result in a error message:

This exercise is intended to illustrate that you should never change the projection of the View! Unless requested otherwise, all data sent to the Field Office is already projected to UTM NAD83. ArcView will allow an image to display in a projected view but it will not display projected vector data correctly unless that data is in Geographic/decimal degrees. This leads to confusion when obtaining data from other sources or using data generated from GPS.

Basic ArcView Different Datum

Dismiss the Projection NAD83 view and activate the Sample Data View. From the sample data/image directory, add the dc.tif image to the View and position it at the bottom of the TOC. Toggle the dc.sid image off. Make the dc.tiff image active then click on the zoom-to -theme button.

With the zoom-in tool. Select and zoom in on the extreme NW corner of the image and try and find the 2 white crosses. They should be visible at a scale of 1:1000. Measure from the NW corner 1400 ft east and 1000 ft south if unable to locate them. Each DOQ image has these “tic”marks in the corner. The solid cross is the location of the NAD83 coordinates. The dashed cross is the location of the NAD27 coordinates. Measure the distance to observe the amount of discrepancy between the two datum. The magnitude of this distance will vary across the country.

This example shows the DOQ in NAD27, while the soils and roads are in NAD83. The offset of man-made linear features are evident in the example. However, only by close examination of the soil and water boundary is there an evident misalignment of the soils layer and the image.

DATUM OFFSET

Point LinePolygon

Raster DataRaster Data is represented by a cell or a group of cells. Each cell contains a

value.

Vertices(x,y)

VectorVector DataVector Data represents objects as exactly as possible such asroads or streams. A vector line is represented by using a series of straight line segments.

POINTS LINES POLYGONS

Attribute DataAttribute Data is the information about a spatial feature. It is also known as “tabular” data. The strength of GIS is the integration of spatial and tabular data.

Soil Attributes Ha

Hb

Mh

The primary attribute data of soils is the map unit symbol which provides the link between spatial and tabular.

TYPES OF GEOSPATIAL DATA

Point - represents an exact feature such as rock outcrops, gravel pits and wells.

Polygon - represents area information such as soils, common land units, and soil survey areas and is composed of multiple straight line segments.

Line - represents linear features such as roads,streams, and pipelines. It may also provide a border to a polygon or separate two polygons.

Advantages of using Shapefiles:

You can create your own data. Edit features in themes. Draw more quickly. As an interchange format.

Vector Data are dominantly in the form of

Basic ArcView - Image Data

In the Arc View Window zoom in on the intersection of two soil map units above field 1. Zoom in until the scale is around 1:100.

Toggle the dc.sid image off and on to view the difference between the compressed sid and uncompressed tiff image. There is an obvious change in the cell values between the two images.

The advantage of the “compressed” image is that it displays more rapidly and requires less disk space.

The compression process reduces the file size of the DOQ imagery. For example the DOQ image for one county in Georgia was reduced from 1.6 Gigabytes to 89.2 Megabytes with a 30 to 1 ratio.

In Summary- What are the issues related to Projections, Map Scale, Data Quality, etc...?

• All data you wish to use together in GIS should be in the same projection, coordinate system and datum.

• Know the limits of your data.-Check with data providers about source scale and

resolution (metadata)

• Is it the “right” data for your business need?• Be aware that misusing data is much easier in digital

format!

USDA National LayersSSURGO-Soil Survey Geospatial Database, NRCSCLU-Common Land Unit, FSA

Other National LayersDOQ-Digital Orthophotography, USGS/FSA/NRCSNWI-National Wetland Inventory, USFWSDRG-Digital Raster Graphs, USGS

State/CountyTransportation/UtilityProperty BoundaryLanduse

Service CenterDistrict Land UnitsJurisdictional Wetlands

<disk drive>:

geodata

<geospatial dataset category (directory/folder)>

soils <geospatial dataset category (directory/folder)>

soil_a_ks057 <geospatial dataset>

<geospatial dataset>

<geospatial dataset>

topographic_images <geospatial dataset category (directory/folder)>

plants

soil_p_ks057

soil_l_ks057

The current standard has 23 common geospatial dataset categories such as soils that consist of 1 or more geospatial datasets.

A geospatial dataset file name should:• be less than 30 characters long• consist of lower case a-z and numerals 0-9• first character always a-z

soil_a_ks057

Elements of a file name

Feature category

Feature type:a-polygon, l-line, p-pointt-table, i-image, etc.

Location: Alpha or numeric FIPS code