Spatial Information Technologies
Watershed Assessment Workshop
April 26-27, 2001
Desktop Mapping for Watershed Assessment
Introduction
“Desktop Mapping” is a narrow description for what now encompasses a broad suite of
spatial information tools used for a variety of watershed management, research, and
planning activities. Spatial information technologies include Geographic Information
Systems (GIS), Global Positioning Systems (GPS), remote sensing, and spatial data
management. Many studies integrate GIS, GPS, and remote sensing with hyrdrologic
models and spatial analyses for watershed assessment, water quality monitoring, wetland
modeling and restoration, groundwater evaluation, decision support, and a wide range of
other hydrologic applications.
Creating a baseline, or “library,” of existing spatial data sets can be the first step in a
watershed assessment. Some of the physical data which exist for most areas include:
topographic information (used to assess the landscape); hydrologic units (hydrologic basins
for relatively large areas); hydrologic data (stream networks); and, soils data (soil types and
permeability). Additionally, images or aerial photographs can be incorporated into the
baseline data to provide a synoptic coverage of the watershed. These data can be used to
provide a spatial reference for air and water quality monitoring sites, assess physical
characteristics of the watershed before field studies, plan field data collection efforts, and
provide input parameters for watershed modeling. Other spatially referenced information,
such as road networks, census data, land ownership, political boundaries, and tax parcels
may be used in conjunction with spatial data layers and field data to support decision
making systems after the assessment.
Workshop participants have already seen how map data can be used to characterize
watershed boundaries and stream networks. The objective of this presentation is to
introduce users to basic capabilities of digital spatial technologies, identify types and
sources of digital data used in watershed assessments, and describe some of the
terminology used with spatial technologies. Data on land use / land cover are used to
demonstrate factors which need to be considered when using existing spatial data sets.
Digital spatial data for the Yates Mill Pond watershed, collected from existing sources, is
shown in a GIS.
This handout outlines several of the topics covered in the presentation and provides
additional information on terminology and data sources.
Geographic Information System (GIS)
Definition
A collection of hardware, software, geographic data, and personnel designed to
capture, store, organize, update, manipulate, analyze, and display geographically
referenced information.
Why Use a GIS?
Maintain and retrieve large quantities of data
Manipulate spatial and related tabular data
Perform complex spatial analysis
Rescale data for analysis
Create thematic maps (maps which depict a specific theme such as “Soil Type”)
Uses of GIS
Hypothesis Generation: This species is found within 100’ of streams.
Analysis: Buffer all streams and overlay the buffers theme with plant locations
theme to identify the number of observations within the buffers.
Communication: You can see on this map that 98% of the observations fall within
100’ of streams.
Overview of GIS Capabilities:
Map production, allowing the user to design and display maps, charts, tables and
images on screen and print maps to a variety of printing and plotting devices
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Integrating maps, images, tabular data, graphics, and charts into
cohesive displays for viewing and evaluation;
Changing map colors and symbols;
Viewing and printing geographic data at user designed scales and
size.
Searching and selecting geographic features of interest based on subject queries,
logical queries, arithmetic queries, and spatial relationships
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Selecting and displaying map features based on location
Analyzing spatial data based on conditions of proximity, adjacency, and
containment
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Interactively measuring distances between mapped locations
Calculating summary statistics such as count, sum, average, and variance
Editing spatial layers and their attribute tables
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Importing spatial layers from a variety of other sources
Creating new spatial layers via. digitizing from the screen or
digitizing tablet
Intersecting and “clipping” of vector spatial layers
Most Common GIS Data Structures
Vector: Point, line, and area features, such as wells, streams, and ponds, which have
discrete positions (or series of positions) stored in an X,Y coordinate system.
Raster: Spatial data stored in a two dimensional matrix or “array,” much like a
checker board. Each raster, or cell, contains a value. Images such as photographs
are in raster format and cells are called “pixels”.
Global Positioning Systems (GPS)
Definition
Satellite system used to record positions (earth coordinates) of point, line, or area
features on the earth’s surface. GPS consists of: Space segment (satellites); control
segment (ground monitoring); and user segment (GPS receivers).
Why Use GPS
Defense: Originally developed, and controlled by, Department of Defense for
submarine navigation
Recreation: Used for hiking, fishing, and hunting
Data collection tool for GIS: Mechanism for capturing, updating, and editing GIS
data
Geographic accuracy: Links spatial data with “the world”
Navigation: Re-locating sample sites; orienteering; driving
Error and Uncertainty in GPS
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Errors result from: ionospheric delays; clock errors (satellite and receiver);
errors in satellite position information; these errors can be corrected by
differential correction
Local errors which affect only the receiver cannot be corrected
Uncertainty arises from poor satellite geometry; total positional error is a
multiple of uncertainty
Typical receiver errors are 30 to 60 feet (10 – 20 m horizontal), without
differential correction; errors are one foot to 15 feet (< 1 m – 5 m
horizontal), with differential correction; vertical errors are typically 2 to 3
times horizontal error
Types of GPS Receivers
Recreational
• Cost range: $100 - $1500
• Accuracies: within 30-60 feet without correction, up to 15 feet with
correction
• Advantages: inexpensive; easy to use; relatively accurate for some
applications
• Disadvantages: limited storage capacity, attributing, and software
interfaces; unable to do post-processing differential correction
Mapping
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Cost range: $1500 - $25,000
Accuracies: within 30-60 feet (horizontal) without correction, up to
15 feet with correction
Advantages: software interface (import/export) with GIS; greater
storage capacity; ability to record multiple attributes; ability to
differentially correct data using post-processing; greater accuracies
with high-end units (1 foot)
Disadvantage: cost; complexity; some units are bulky.
Survey
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Cost: > $25,000
Accuracies: less than 1 cm horizontal; up to 1 cm vertical
Advantage: very high accuracy
Disadvantages: cost; complexity; longer times required to collect
data
Remote Sensing
Definition
The art and science of obtaining information about an object or feature without
being in direct physical contact with it. Conventionally refers to recording
electromagnetic radiation using aerial photography for visual analysis and digital
(non-photographic) imagery for computer-aided analysis.
Why Use Remote Sensing
Detail
Synoptic / complete view
Historical record
Reduce field work
Rapid means of getting information
Three dimensional viewing
Types of Remotely Sensed Images / Processing
Conventional aerial photography is acquired, usually from aircraft, using a large
format camera with film. Photographic sensors (film) are used to detect
electromagnetic radiation which has wavelengths .9m (900 nm).
Non-photographic imagery is acquired from aircraft or spacecraft using scanners or
other devices in which the sensing medium is not film. Reflectance or emission of
electromagnetic radiation is recorded as a number. Devices have been developed to
detect virtually any portion, or portions, of the electromagnetic spectrum.
Softcopy photogrammetry, also called digital photogrammetry, is the analysis of
digital images which have been acquired using film. “Hardcopy” photographs are
scanned and analyzed digitally using a variety of techniques which combine
conventional (visual/manual) analyses and digital (non-photographic/automated)
techniques.
Land Use / Land Cover Classification
Definition
Land use refers to activities which are occurring on the land; generally, refers to
how land is utilized by humans. “Residential area” and “camp ground” are land
uses.
Land cover refers to features, objects, or material which cover the surface of the
earth. “Asphalt” and “Forest land” are land covers.
Criteria for Land Use and Land Cover Classifications
Classification system should be widely applicable / apply to broad geographic areas
Categories should be hierarchical; classes can be sub-divided and aggregated
Classes are mutually exclusive; a given area cannot be in more than one class
Classes should be well defined
Level of detail should be consistent across categories
Should be able to achieve relatively high thematic accuracies (85% in each
category)
Standard Land Cover Classification Systems
National Land Cover Data (NLCD) (also known as MRLC data) – One of the
projects sponsored by the MRLC Consortium* was production of land cover data
for the conterminous. United States. Land cover was mapped using general land
cover classes based on the Anderson (USGS) Classification System. For example,
forest is classified as either deciduous, evergreen or mixed. Land cover
classification was based on Landsat 5 Thematic Mapper ™ satellite data and a host
of ancillary sources. It is not appropriate to attempt to derive detailed classes using
Landsat TM data due to issues of spatial resolution and interpretability of data.
Thus, no attempt was made to derive classes that were extremely difficult or
“impractical” to obtain using Landsat TM data, such as the Level III urban classes.
The MultiResolution Land Characteristics (MRLC) Consortium was sponsored
originally by the U.S. Geological Survey (USGS) National Mapping Division
(NMD) Earth Resources Observation Systems (EROS) Data Center (EDC), the U.S.
Environmental Protection Agency (EPA) Office of Research and Development
(ORD) National Exposure Research Laboratory (NERL) Environmental Science
Division (ESD) Landscape Characterization Branch (LCB), the National
Oceanographic and Atmospheric Administration (NOAA), and the U.S. Forest
Service (USFS). MRLC Consortium continues to thrive, as evidenced by new
sponsors such as the National Atmospheric and Space Administration (NASA) and
the Bureau of Land Management (BLM).
National Vegetation Classification System is an FGDC standard through the first 5,
physiognomic, levels. Work continues through the USGS-NPS Vegetation Mapping
Program, in association with several public and private agencies, on the floristic
levels (lower 2 levels) of the vegetation classification standards. The lower levels
of the Classification have several thousand cover types and associations to date.
Work will continue for an indefinite period as new classes are proposed and
existing classes are refined or aggregated. This system applies to classification of
just vegetation, not general land cover categories.
Classification of Wetlands and Deepwater Habitats (also known as NWI
classification system) is a system which has been used by the US Fish and Wildlife
Service (USFWS) to categorize all wetlands areas in the US (Cowardin et al. 1979).
The National Wetlands Inventory (NWI) mapping was based primarily on
interpretation of high altitude color infrared photography, although collateral data
such as soil surveys, nautical charts, and field checks were also used in some areas.
Wetland polygons produced from the photo interpretation were mapped using the
USGS 7.5 minute quadrangles as a base and are available, by quad, from the
USFWS. (Data have also been put in digital form). This system has been adopted as
an FGDC standard for classifying wetlands.
Land-Based Classification Standards (LBCS) were developed by the Technical
Advisory Panel of the American Planning Association and provide a consistent
model for classifying land uses based on their characteristics. The standards are
based on a multi-dimensional land-use classification model which extends the
notion of classifying land uses by refining traditional categories into multiple
dimensions, such as activities, functions, building types, site development character,
and ownership constraints. Each dimension has its own set of categories and
subcategories. These multiple dimensions allow users to have precise control over
land-use classifications.
Status of Land use / Land Cover Classification Systems
Currently, there have been national standards developed for cadastral data, wetlands
classification (NWI), and vegetation classification (NVCS). There are no national
standards for classifying land cover, although the NLCD (MRLC) classification
may serve as a national model. Land use also does not have a national standard for
classification although the LBCS was designed for compatibility with the overall
framework set forth by the FGDC and LBCS standards were developed under the
FGDC framework. The APA has proposed that it be adopted as a national standard
for land use classification.
Federal Geographical Data Committee
Description
The Federal Geographic Data Committee coordinates the development of the
National Spatial Data Infrastructure (NSDI). The NSDI encompasses policies,
standards, and procedures for organizations to cooperatively produce and share
geographic data. The 17 federal agencies that make up the FGDC are developing
geospatial data standards for implementing the NSDI in cooperation with
organizations from state and local governments, the academic community, the
private sector, and, to the extent feasible, the international community.
The National Spatial Data Infrastructure
Consistent means to share geographic data among all users could produce
significant savings for data collection and use and enhance decision making.
Executive Order 129906 calls for the establishment of the National Spatial Data
Infrastructure defined as the technologies, policies, and people necessary to
promote sharing of geospatial data throughout all levels of government, the private
and non-profit sectors, and the academic community.
Metadata
Description
Metadata, or “data about data,” describe the content, quality, condition, and other
characteristics of data. The Federal Geographic Data Committee approved the
Content Standard for Digital Geospatial Metadata in June 1998.
General Contents for Metadata
Identification Information: basic information about the data set. Examples include
title, geographic area covered, currentness, and rules for acquiring or using the data.
Data Quality Information: an assessment of the quality of the data set. Examples
included positional and attribute accuracy, completeness, consistency, sources of
information, and methods used to produce the data.
Spatial Data Organization Information: the mechanism used to represent spatial
information in the data set. Examples include the method used to represent spatial
positions directly (such as raster or vector) and indirectly (such as street addresses
or county codes) and the number of spatial objects in the data set.
Spatial Reference Information: description of the reference frame for, and means of
encoding, coordinates in the data set. Examples include the name of and parameters
for map projections or grid coordinate systems, horizontal and vertical datums, and
the coordinate system resolution.
Entity and Attribute Information: information about the content of the data set,
including the entity types and their attributes and the domains from which attribute
values may be assigned. Examples include the names and definitions of features,
attributes, and attribute values.
Distribution Information: information about obtaining the data set. Examples
include a contact for the distributor, available formats, information about how to
obtain data sets online or on physical media, and fees for the data.
Metadata Reference Information: information on the currentness of the metadata
and the responsible party. The standard has sections that specify contact
information for organizations or individuals that developed or distribute the data
set, temporal information for time periods covered by the data set, and citation
information for the data set and information sources from which the data were
derived.
Factors to Consider When Acquiring or Using Spatial Data
General Factors
Access to data
Data restrictions
File format
Storage format / tiling scheme
File size
Project Specific Factors
Extent of Coverage
Projection, units, and datum
Scale
Resolution
Currency / con-currency
Thematic (feature) accuracy
Positional accuracy
Attributes / feature data
World Wide Web: Sources of Data
There are a multitude of public and private agencies or organizations which maintain web
sites showing the availability of existing spatial data sets and/or imagery and photography.
There are also numerous vendors maintaining web sites for spatial hydrology or hydrologic
software which links to spatial formats. The following list is not intended to be
comprehensive, but may provide a starting point for exploring data availability, especially
for watershed assessment.
GIS / Map Data and Information
http://www.lib.ncsu.edu/stacks/gis/webmap NCSU libraries maintains a spatial
data library; this site provides links to a number of local, regional, state, and
national sources of data.
http://www.cgia.state.nc.us/cgia/basinpro.html A desktop GIS project that
features a comprehensive set of N.C. geospatial data and selected tools for water
quality analysis and watershed planning.
http://fgdc.er.usgs.gov/ Federal Geographic Data Committee
http://ask.usgs.gov/ U.S. Geological Survey Earth Science Information Center
http://gisstore.esri.com/ ESRI site for software and publications
http://www.gisdatadepot.com/ repository for free geospatial data
http://www.ncmapnet.com/ portion of North Carolina’s Corporate Geographic
Database
http://www.igif.org/ The International Geographic Information Foundation, “a
Non-Profit Charitable Foundation dedicated to the improvement of the quality of
life on Earth through better use of geographic information”
http://www.tenlinks.com/MapGIS/ provides links to data resources, training,
services, etc.
http://gis.enr.state.nc.us/d_default.htm Information about GIS activity at the NC
Department of Environment and Natural Resources (DENR)
http://cgia.cgia.state.nc.us:80/cgia/ Center for Geographic Information and
Analysis, NC’s repository for geospatial data
GPS Data and Information:
http://ares.redsword.com/gps/old/gps_vnd.htm#commercial GPS equipment
manufacturers
http://www.cmtinc.com/gpsbook/index.htm GPS tutorial; Covallis
Microtechnology, Inc.
http://www.trimble.com/gps/index.htm GPS tutorial; Trimble, Inc.
http://www.gpsworld.com/ Journal and links to other GPS sites
http://www.ngs.noaa.gov/ National Geodetic Survey
http://www.ncgs.state.nc.us/ North Carolina Geodetic Survey
Remote Sensing Data and Information
http://earthobservatory.nasa.gov/ NASA site which provides a freely-accessible
satellite imagery and scientific information about our home planet.
http://eos.gsfc.nasa.gov/ Information on NASA’s Earth Observing Systems
satellite program and data
http://www.casi.ca/listof.htm Canadian Remote Sensing Society’s list of remote
sensing and GIS web sites
http://geo.arc.nasa.gov/sge/landsat/landsat.htm Access to Landsat information /
ordering
http://terraserver.homeadvisor.msn.com/default.asp locate National Aerial
Photography
http://www.spaceimaging.com/index.htm Space Imaging Corp., markets satellite
and aerial imagery
http://www.earthdata.com/index2.htm EarthData, markets image and GIS
products and services
http://ask.usgs.gov/photos.html USGS site for locating and ordering aerial
photography
Land Cover Classification
http://biology.usgs.gov/npsveg/ National Vegetation Classification System
http://www.planning.org/lbcs/index.html Land Based Classification System
http://www.nwi.fws.gov/ US Fish and Wildlife Service National Wetlands
Inventory
http://www.epa.gov/mrlc/text.html MRCL/NLCD information and data
Watershed Assessment / Spatial Data
http://www.spatialhydrology.com/ Provides links to other resources; Journal of
Spatial Hydrology (coming in June)
http://civil.ce.utexas.edu/prof/olivera/esri98/p400.htm ArcView scripts
developed to extract hydrologic, topographic and topologic information from digital
spatial data of a hydrologic system, and to prepare an input file for the Hydrologic
Modeling System
http://www.rkrhess.com/swrm.html Stormwater modeling system
http://www.cgia.state.nc.us/cgia/basinpro.html A desktop GIS project that
features a comprehensive set of N.C. geospatial data and selected tools for water
quality analysis and watershed planning.
http://water.usgs.gov/wsc/index.html USGS watershed assessment site with links
http://www.wes.army.mil/el/wq/00abstracts.html Water 2000 conference
abstracts
http://www.epa.gov/ow/compendium/basins.htm system for environmental
analysis developed by EPA for integrative watershed and water quality-based
assessments.
Abbreviated Glossary of Terms
Absolute location – Map or earth location associated with a specific set of coordinates
Almanac – file containing parameters for general location of GPS satellites; used for premission planning to determine satellite availability in a given geographic area
Attribute – descriptors of point, line, and area features in a GIS
Attribute Table – a table (much like a spreadsheet) that contains attribute information
linked to a spatial feature
Autonomous positioning – uncorrected GPS data
Coordinate System – grid overlay used to locate positions on a map. The most common
coordinate systems are latitude / longitude, state plane, and UTM.
Coverage – a single thematic map in a multimap GIS database; also called data layer
Crosswalk – process of matching disparate categories between classification systems or
maps
Datum – a network of control points used to derive a mathematical model of the earth’s
surface. Selection of datum is very important since each datum has a different point of
origin and may define a different ellipsoid(s). The North American Datum 1983 (NAD83),
continental U.S. (Conus), is most commonly used for state projects.
Differential Correction – Technique for determining and applying a correction factor to
GPS data collected in the field by comparing them to data collected at a known point.
Dilution of Precision (DOPS) – uncertainty in position due to geometry or arrangement of
satellites; most important is usually PDOPs (positional dilution of precision)
Ellipsoid – a smooth elliptical model of the earth’s surface; X,Y (horizontal coordinates)
are referenced to an ellipsoid
Geoid – the surface that the sea would assume if it covered the earth; surface of equal
gravitational attraction; meal sea level (note: sea level isn’t “level”). Z (vertical coordinate;
elevation) is referenced to the geoid.
Label (Annotation) – text information used to identify a feature or location in a spatial layer
Line / Arc – a series of connecting X,Y positions (length, and no area)
Node – 1) the end or beginning point of a line. 2) an intersection of two or more lines
(similar to a point)
Pixel – a single cell in a raster image, corresponding to a ground area; “picture element”
Point – a single location having an X,Y (and sometimes – Z) position (no area, or length)
Polygon – one or more connecting lines that form a single, closed, spatial feature (area and
perimeter)
Projection – representation of the spherical globe on a flat surface; the three basic
representations are cone, cylinder, and plane. NC topographic maps use a Lambert
Conformal Conic projection which is also the same projection used for most state projects
(state plane “projection”). Many federal projects use a Universal Transverse Mercator
projection (cylindrical).
Relative accuracy – positional accuracy determined in relation to a second object or feature
Resolution – size of the smallest object that is sharply defined on a photograph or map; the
amount of ground area covered by a single pixel in a raster image or GIS coverage
Scale – the ratio of a distance measured on a map to the corresponding distance measured
on the ground
Tic (Tick Mark) –a registration (known) location used to align maps during digitization
Waypoint – a point of known position used in navigation.