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If you haven’t noticed, the USGS has updated its website. As a result, elevation data in GeoTiff format and land cover data from 1992 are no longer available. Why it that important? AERMAP, the EPA program to generate terrain elevations and effective terrain heights for use with AERMOD, requires data be in GeoTiff. Also, AERSURFACE, which calculates meteorological surface characteristics from land cover data, requires the 1992 national land cover dataset (NLCD) for input. USGS did not see enough use of the GeoTiff elevation data and the 1992 NLCD to justify keeping it available.
At NaviKnow, being billionaires is not our goal (I wish), but we do strive to provide information and tools that are useful to the air quality community. Since USGS is no longer providing the data necessary for dispersion modeling with AERMOD, NaviKnow is making these data available to the air quality community, in the format needed, free of charge. Remember, we are not out to be billionaires.
EPA has provided a workaround by providing links to programs that can convert elevation data files from USGS to GeoTiff, but those tools require additional time, effort, and expertise to use. Also, though EPA has made available the 1992 NLCD, ask yourself, how much longer are you going to use obsolete data that is supposedly “critical in determining the boundary layer characteristics” (40 CFR 51 Appendix W, 18.104.22.168) for your air quality analysis?
Regarding the elevation data, NaviKnow has already converted all the data, so you can point and click to download the file(s) needed, in GeoTiff format. Starting in February 2019, NaviKnow is rolling out the NaviKnow (NK) NED Tool online. Nothing to install. Open your web browser, log in, zoom to the area you are interested in, then click to download the files. It is that simple.
Regarding the 1992 NLCD, NaviKnow has decided to let those data stay put, unless the data are temporally appropriate for your project. In keeping with our motto, “providing the information you need and the tools to put it into action”, NaviKnow will be rolling out an AERSURFACE-like tool with data from 1992, 2001, 2006, 2011, and 2016 integrated into the workings of the tool. The same algorithms in the current version (version 1301) AERSURFACE are used with a few adjustments to roughness length values (see our proposal on the adjusted values). This tool will be available in February-March 2019.
If you have stock piled elevation data files or have found some DEMs on some website, keep in mind that data do change over time, even terrain data. There may be instances where the DEMs from yesteryear are obsolete and are not representative of the area they are for.
In testing the NaviKnow NED Tool, we did see differences in elevation values (provided by AERMAP) between data obtained from USGS mid-2018 (in GeoTiff) and data recently obtained (January 2019) and converted to GeoTiff. We performed a point by point comparison often of thousands of points. For the most part, the differences were tiny (<< 1%), which is to be expected. However, we did find instances where the differences where more significant (> 10%).
Below is imagery of an area typical of where significant differences in elevation were found, between data obtained few months ago and data obtained a few weeks ago. Notice the two bright green control points. One point had an elevation difference of < 0.01% and the other a 15% difference between the two data sets.
Below is the same area represented by the NED data from several months ago. The color shading depicts the elevation values. You should be able to make out the spill basin at the base of the dam shown and stream of water released from the dam. There isn’t much variation in the elevation since the colors are so similar.
In the image below, it is the same area, but notice there is more variation in the colors depicting elevation values. The exact same color scale was used between the images above and below. Though the resolution of the data is the same (1/3 arc second) for both data sets, there appears to be better accuracy of the source data gathered in the more recent data set. Notice the road leading the dam is visible in the more recent data but not the older data.
In reviewing the metadata for the two data sets, the data from months ago was published in 1999 but the data from a week ago was published in 2017. Though the metadata did not go into detail on the topic, most likely, more modern and more sensitive instrumentation was used to gather the newer data.
In keeping with EPA guidance in using the most recent and readily available meteorological and monitoring data for your air quality analysis, it would be consistent to use the most recent and readily available terrain data. If you choose to use DEM data from decades past, keep in mind these data may not be representative (at most) and may open you up to a battery of questions to defend your selection of data (at least) if the modeling is being challenged in court.
Starting in February 2019, NaviKnow will provide the means for dispersion modelers to download the most recent terrain data in GeoTiff format available anywhere. No need to convert files from USGS to GeoTiff yourself. We have done all that for you.
Later in 2019, we will have the NaviKnow Aersurface tool, to perform an AERSURFACE-like analysis using the most recent and readily available land cover data or temporally representative land cover data for your project. No reason for you to go to extra effort. NaviKnow has taken care of the heavy lifting.
If you found this article informative, there is more helpful and actionable information for you. Go to http://learn.naviknow.com to see a list of past webinar mini-courses. Every Wednesday (Webinar Wednesday), NaviKnow is offering FREE webinar mini-courses on topics related to air quality dispersion modeling and air quality permitting. We also have articles air quality issues at http://naviknow.com/news. If you want to be on our email list, drop me a line at firstname.lastname@example.org.
One of the goals of NaviKnow is to create an air quality professional community to share ideas and helpful hints like those covered in this article. So if you found this article helpful, please share with a colleague.
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