Tornadoes in Butler County, Iowa in 2011
The Tornado History Project generates approximate paths through separate historical archives provided by the Storm Prediction Center (SPC) and the National Climatic Data Center (NCDC). Paths generated from the NCDC data are typically more detailed, but neither archive gives exact path information. There are several reasons that paths may be inaccurate:
- Tornado touchdown and liftoff coordinates were recorded with only 2 digits of decimal precision (i.e. [33.72, -86.15] vs [33.71689, -86.15463]). As a consequence, the observed points on the map may be slightly off from actual. Note that beginning in 2009, up to 4 digits of decimal precision are given.
- Coordinates have not historically been calculated via GPS (Global Positioning System). Thus, tornado touchdown and liftoff coordinates should be considered as estimates only.
- Tornadoes may not have been in contact with the ground for the entire path as depicted on the map. Storm damage in any location depicted under the "path" should not be inferred.
- Although paths are drawn as straight lines between any two sets of coordinates, the tornado may have "zigzagged" in some way.
- Although paths are drawn as uniform thin lines on the map, it is likely that the tornado changed size over its lifetime. Damage in any location depicted under (or not under) the "path" should not be inferred.
Even with the above in mind, the data is the best available. If you see a path that is depicted incorrectly, please post a comment in the tornado forum indicating why you belive the path to be incorrect.
Paths generated from NCDC data are typically more detailed due to the way coordinates are listed in each archive:
- SPC database - A maximum of 2 coordinates (touchdown and liftoff) is given for each tornado, or each state segment of a tornado if it is a multi-state tornado. Thus the vast majority of paths are depicted as simple straight lines.
- NCDC database - Some tornadoes have mutliple sets of coordinates within any state, generally corresponding to entry and exit points by county. Thus, since more coordinates have been used to draw the path, the depicted path should be more accurate. Having said that, most tornadoes do not have additional path data in the NCDC archive. When they do, paths are drawn from the NCDC data.