Introduction to Geographic Information Systems in Forest Resources
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Introduction to Geographic Information Systems in Forest Resources |
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Discussion:
The web abounds with GIS data. Finding the specific data you need may not be too easy, but the journey is part of the adventure.
GeoSpatial Data Clearinghouse (FGDC-Compliant Metadata)
Washington State GeoSpatial Data Archive
: If you obtain data from the web, or from any source, make sure you know its ultimate source. For example, many of the data sets at the UW Map Library Geospatial Data Archive are originally USGS data. When referring to these data sets, give credit to the USGS, not the UW Map Library Geospatial Data Archive! When using data that you found on the Clearinghouse, give credit to the ultimate source, not the Clearinghouse node!
The single best tool to use for finding all various types of data on the net is the Web Search Engine.
Search engines allow you to look across the web for keywords. Use the search engines to look up keywords like "GIS Data."
Here is a list of some common search engines (which I found by searching a search engine on the term "search engine").
My current personal favorites are Ixquick, Dogpile, and Metacrawler, which simultaneously search multiple search engines.
Here are a few references on search engines. These sources can teach you techniques to help narrow your searches down so that you limit superfluous hits.
Lynch, Clifford. March 1997. Searching
the Internet, Scientific American.
Phelps, Alan. 1999.
Looking for Answers. Smart Computing in Plain English. 7(11).
The Spider's Apprentice: A Helpful
Guide to Web Search Engines
How search engines
work.
How search engines
rank web pages.
Search engine
features chart
How big are the search
engines?
Power Searching 101:
a tutorial.
Web search engines are good at finding data in general, but they are poor at finding context-specific content. If you do search for "GIS Data" using a search engine, you will get numerous hits. However, you will need to sort through those many hits to find data for the place you are interested in. The reason for this poor performance is that the search engine spiders crawl across pages and save keywords. The keywords may or may not be associated with a page that really contains the data you want. The search is just for a word, but the word is not placed in a specific context. Many matches for "GIS Data" will be pages that do not contain any links to real GIS data!
Enhanced GIS data searching is done using the GeoSpatial Data Clearinghouse.
GeoSpatial Data Clearinghouse
The primary method of searching for GIS data on the Net is to use the National Spatial Data Infrastructure's series of servers known collectively as the Geospatial Data Clearinghouse. These servers generally do not house spatial data sets, but they contain metadata, which are files that describe geospatial data sets, giving various pieces of information, such as spatial extent, theme keywords, points of contact, etc. Once you find metadata for a data set you are interested in, you can use the contact information contained within the metadata record to find out how to get a copy of the actual data. Presently, all data sets created using US federal funds are required to also have FGDC-compliant metadata records.
Our Washington State Node of the NSDI Clearinghouse has an on-line system for contributing metadata. If you generate any unique data that are geospatial in nature, you can submit metadata to the server.
Metadata are the files created by data developers that contain detailed descriptions of the data sets. Metadata records will generally contain information such as the title, data format, developer, time of creation, source scale, projection & coordinate description for a GIS data set. A specific content standard exists, which requires all compliant metadata to be formatted in the same way, with a core set of values, as well as an extended optional set of values. The standard has been developed by the Federal Geographic Data Committee (FGDC), so these metadata records are known as FGDC-Compliant metadata.
The content standard is the reason the Clearinghouse is effective as a search engine. Because the file structure and content of metadata records is standardized, searchable databases can be built from the metadata records. This allows for true database queries on the metadata.
For example, if you may be searching for raster digital elevation data. Looking for "elevation" in a web search engine will result in a wide variety of hits, most of which will have little to do with your needs. Using the Clearinghouse, it is possible to search only metadata records (rather than all web pages in general).
It is possible to limit the search to a geographic extent by place name or by bounding coordinates:
It is possible to specify a time period which the data set represents, and to search for the keyword "grid" anywhere in the metadata record, and to limit the search to records that contain the string "elevation" in the title of the data set:
Because a series of servers exists, each one housing metadata for a particular organization or geographic region, it is possible to narrow down the search for "local" data sets:
Finally, the search is submitted, and after a few moments, the results are displayed:
The individual records can be viewed:
And then the full record for each data set can be browsed. When you see metadata in this format, it should be FGDC-compliant, and contain values for specific fields, such as projection, coordinate system, units, data format, and a person to contact in order to obtain the data set. Sometimes the metadata record also contains an online linkage, that will allow you to download from a link in the record itself.
If you are interested in getting copies of the data sets, you follow the link to Distribution Information:
Here is a link to the complete metadata record for this data set.
If you find metadata for a geospatial data set, and the metadata record does not look like this, with this particular format and these particular fields, you do not have FGDC-compliant metadata.
Since its foundation in 1879, the United States Geological Survey has been a leader in developing map data for the USA. Much of the map data developed by the USGS has been converted to digital formats that can be imported into GIS software. Nearly all of these data sets are available for free via the Net. Some data sets are not available online, but may be available at low cost by ordering directly from the USGS, or the data sets may be available offline for free at a depository library such as the University of Washington Map Library.
Data sets that the USGS provides online at no cost can be found at the USGS GeoData web site.
A very large variety of data at many different source scales is available here. Most of the data are vector versions of the same data shown on the USGS quadrangle maps (1:24,000; 1:100,000; 1:2,000,000; etc.). In addition to the vector data sets, a large assortment of DEMs are available.
Many of the data sets that are available need special translation software in order to be imported into GIS software. Most GIS software comes with import and export utilities for these formats. However, ArcView does not include an SDTS format importer.
In order to import SDTS vector data to ArcView, you must either use SDTSIMPORT in ArcInfo to create ArcInfo coverages, or sdts2dxf.exe from the BLM's GIS ftp site to create AutoCAD DXF files.
To import SDTS raster DEMs, use the SDTS DEM to ArcView Grid Conversion Utility to import directly into ArcView or the sdts2dem.exe from the BLM's GIS ftp site.
The U of W Map Library has a large collection of GIS data, including City of Seattle data, Statewide data, including DEMs, digital orthophotos (DOQQs), and digital raster graphics (DRGs - scanned and geo-referenced versions of USGS quadrangle maps).
Some of the data sets, including the City of Seattle data, are restricted to Internet connections originating within the U of W's IP addresses, and are not available for general public download. Other data sets, such as the 10 m DEMs, are available for free without restriction.
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The University of Washington Spatial Technology, GIS, and Remote Sensing Page is provided by the College of Forest Resources and the College of Ocean and Fisheries Sciences through Unit-Specific UIF. Site administrator: Phil Hurvitz. |
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