Philadelphia Water Department
Office of Watersheds
Green Cities,
Clean Waters
Here you'll find answers to the frequently asked questions (FAQ).
We've also included very basic tutorials for Google Earth and MapWindow GIS to get you up and running. They are intended to give you a start and perhaps clear a few of the hurdles a new user will typically encounter. They are not nearly as detailed as the excellent tutorials and user information that is available on the programs' respective websites:
Both of these programs have a lot of capabilities, and the stormwater basin inventory information is just scratching the surface of what these tools can do. The best way to learn more is to use them.
Frequently Asked Questions (FAQ)
What is orthophotography? / What is an "ortho"?
Where can I get aerial photography for my area?
How do I use the data on the CD-ROM?
What is a shapefile? / What is the file with extension .dbf, .shx, .prj?
What is a KML file? / What is a KMZ file?
Why does program XYZ crash or not work on my computer?
Are there any options for using GIS on Mac / Linux?
Where can I get more information about retrofitting stormwater basins?
What is GIS?
GIS stands for Geographic Information System. At its most basic, a GIS can be thought of as a way of keeping track of where things are in the real world. GIS ranges in complexity and cost from free basic viewer software to Enterprise GIS databases and servers that can store and serve terabytes of information. A GIS gives us the ability to organize spatial and descriptional information about features in the real world.
More information at Wikipedia
What is a Projection?
Most of the time when we are working with geographic features, we are looking at a 2D or flat surface, such as a map or a computer display. However, the Earth is not flat (It isn't exactly round either, but that is another topic). Representing a curved surface like the Earth on a flat surface is a bit like peeling an orange and pressing the skin flat. You need to make multiple cuts, and it is still difficult to get a large piece to lay exactly flat. In order to better represent the area of interest, geographers project features on the curved surface of the earth onto a flat surface.
In southeastern Pennsylvania, we use a projection that is designed to make our part of the world stand out and be as flat and usable as possible. This projection is called Pennsylvania State Plane 1983 South. The base unit of measurement is U.S. feet, and essentially establishes a rectangular grid large enough to contain the entire southern half of the Commonwealth. If your organization uses an existing GIS system, it is likely that you use this same projection. If not, data can be re-projected using projection utilities built into commercial GIS software or free software such as The U.S. Army Corps of Engineers CORPSCON Software.
More information at Wikipedia
What is orthophotography? / What is an "ortho"?
An orthophotograph is a photograph that has been digitally corrected to be "flat" and usable as a map. The image is a photographic representation of the earth's surface. These Images have the ability to orient the user in a very visual, familiar way. Images are limited, however, by their resolution, or the amount of the real world that can be represented by individual pixels. One disavantage of these "bitmap" or "raster" images is that you can only zoom in so close before the image becomes distorted due to lack of resolution. Some types of images may also be compressed to reduce file size.
More about orthophotography at Wikipedia.
Where can I get aerial photography for my area?
There are many sources of this information. 2004 Color orthophotography (see above for definition) is available from the Pennsylvania Geospatial Data Clearinghouse PASDA website. Area 9, including 90% of the of the Montgomery County portion of Wissahickon Creek Watershed, is included on this disc. Those who would like the complete set of aerial photography should download Area 4, Area 5, and Area 8 (unfortunately not available at the time of writing) as well.
How do I use the data on the CD-ROM"?
This CD-ROM contains data in two primary formats, shapefiles and Google Earth KML files. We have included software applications for viewing and working with these data. Read about these file types below.
What is a shapefile? / What is the file with extension .dbf, .shx, .prj?
Shapefiles are the most common form of vector data used in traditional GIS, and they require a number of accompanying "helper files" to work correctly. You will encounter at least three and as many as eight or more files, such as shapefilename.shx, shapefilename.dbf, shapefilename.prj, shapefilename.sbn, or shapefilename.sbx. These "helper" files may store descriptional information, keep track of how to draw the features, define the map projection, etc. Because there are so many files to keep organized, these files are combined with the shapefile itself (shapefilename.shp) and compressed in a .zip archive. The .zip archive can be unzipped to extract the necessary files, using the free 7-zip software, or another file compression utility such as WinZip (not free).
More Information about shapefiles on Wikipedia
Official shapefile file extensions help from ESRI
What is KML file? / What is a KMZ file?
KML, or Keyhole Markup Language, files are used by Google Earth and many other new web based mapping tools. Many of the informational data sets included on this CD-ROM are available as KML files for viewing in Google Earth. By customizing KML files, we can change the appearance of symbols and add descriptional information. KML files are human readable and based on XML and therefore very easy to use and customize.
Official KML documentation from Google
Why does program XYZ crash or not work on my computer?
All of the programs included on this CD-ROM have minimum hardware requirements in order to run. PWD is not able to provide technical support on an individual basis or diagnose your particular difficulty. We recommend that you consult the documentation from the links on the Software Page. Note that Google Earth and other web based mapping tools require a broadband internet connection and a modern PC with sufficient video memory to run smoothly. Some of the data layers on this CD-ROM, such as orthophoto images, are very large and will redraw somewhat slowly. When working with a GIS application that is drawing large datasets, the user should refrain from issuing too many instructions to the computer while it is drawing or processing requests.
Are there any options for using GIS on a Mac?
Google Earth runs well on Macintosh computers that meet the program's hardware requirements, and there is also support for Linux. For more traditional GIS, we recommend Quantum GIS, a very good cross-platform open source GIS. This application is capable of perfoming most of the same tasks described in the MapWindow excercises, with the exception of "clipping" shapefiles.
Where can I get more information about retrofitting stormwater basins?
There is a lot of great information in the PWD Stormwater Management Guidance Manual.
You can get more information and view a video about a retrofitted basin on GreenworksTV (streaming .ram video file)