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Nov 08 2011

Virtual Site Visit 2

If you want to have a quick look at a project site without actually going there, the free applications I mentioned in the last post will certainly get the job done.  However, it is often important to gather more information that requires better data and more powerful analysis tools. 

bangkok

Imagery and Topography

There are limitless types of data someone might have a need for and could possibly find online.  In the U.S. we have an enormous amount of data available for free.   For this example I will focus on the two primary pieces of data I use, aerial photography and digital terrain info.

For any type of data I would recommend familiarizing yourself with a few pieces of key information.  These include;

  • Data Format
  • Type
  • When and How it was Acquired
  • Coordinates and Datum
  • Accuracy or Resolution

If you have a grasp of this info you will be in a position to use the data more efficiently, how to best use it, and be able to know the limits your data may have.  Below is a table I put together a while ago that helps summarize this info.

key info

Accuracy/Resolution

Images come in different resolutions, often reported in pixel size.  An aerial image is made of squares (pixels) and each square has one color.  The smaller the pixel size, the better the image quality.  As a base line, you can get 1 m color imagery for anywhere in the U.S. sometime in the last 3-4 years.  The best aerial imagery currently available, usually not free, is 3” pixel.  If you need more resolution and as recent as possible you will have to do a little more digging to see what is available.  Here is a good visual showing what the different resolutions look like.

pixels

Elevation data is similar in it’s range of accuracy.  Elevation data comes in a gridded format or a triangulated irregular network (.tin).  Gridded formats will have a point spacing associated with them.  Just like images, the tighter the point spacing the better representation of the actual terrain.  Contour interval is another often reported item that helps us understand the vertical accuracy of the topographic data.  USGS contour maps typically have 40 ft or 20 ft contours.  The vertical map accuracy standard (VMAS) for these maps is that no less than 90% of the data will be within half of the contour interval of the actual elevation.  So for 20 ft contours most of the data will be within 10 feet of the actual elevation.  Some of it will be closer, but that is the margin of error reported.  Here is a table that provides the VMAS info, from the National Standard for Spatial Data Accuracy (NSSDA) ;

Map Accuracy

Digital Elevation Models (DEM) are available for the U.S. in various levels of accuracy.  You can find 30 meter DEM with a 40 ft contour interval for the entire U.S., and there is often better terrain data available.  The best topographic data available is the 1 ft contour interval Here is a visual that of the different levels of DEM data available.

terrain

Data Sources

There are a lot of places to obtain data.  I will share the major ones I use, but the best data is usually obtained from local state, county, or city resources.

  • The USGS Seamless Server is a good place to start.  You can find more than imagery and topographic data here.  Most of the data they display is free to download, though there are some file size limitations so for larger areas this might not be the best place to go to.

seamless

  • The USDA Geospatial Data Gateway is another great resource.  Their data is packaged based on counties.  There are some similar data sets that the USGS offers, but you can get larger pieces at a time from the USDA.

USDA

  • If you want more detailed data check and see if you State, County, or City has a good GIS program and has free data available online.  This varies drastically across the country.  Here are some examples from Northern Colorado;

Larimer County Land Explorer

– City of Greeley GIS

City of Fort Collins GIS Data

  • For other types of nation wide data, a good place to start searching is the Geo.Data.Gov site.

geodatagov

  • If you need really detailed and recent data, you will likely have to pay for it Smile.

Tools

Once you actually get the data you want you need to have the right tools to be able to use it as you need and extract and/or display what you want.  I have written about GlobalMapper before, and I think it is a must have when working with geospatial data sets.  The other two pieces of software to think about are a GIS program and a CAD program.  Both GIS and CAD have their pros and cons.  I keep hoping one or the other will figure out what is missing and I will only need one of these in the future, but until then.  I often use all three types of software on most projects.

Any software that you use to help analyze project sites?  

2 comments

  1. cameron jenkins

    I have found the following website very useful in explaining where to obtain imagery, topography, and other data sets.

    http://xmswiki.com/xms/GSDA:GSDA

  2. Glenn Fischer

    This will likely be a shot in the dark for most projects, but http://www.opentopography.org/index.php is a site where you can search for and download free LiDAR datasets provided by the community.

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