The problem with trying to extract clutter height from the SRTM data is that you probably will only get about the 70 percentile or so of the real heights (this depends on the density of the clutter in any particular area). Remember the SRTM data relied on the RADAR and LIDAR echo returns to establish height. It had to hit clutter/tree density that was enough to create that echo return, tops of trees are not usually dense enough so trying to extract between the two sets has proven to not be accurate enough. Believe me I spent a lot of time trying that process. I have another paper that I will send you where I went through that drill. Also relying on that data at 30 meter resolution to compare to the 10 meter resolution NED data will have you using the SRTM data that has a lot of assumed interpolated height values. With closer spaced actual data at 10 meter spacing there is much less interpolation and a lot less terrain averaging. This becomes quite critical in areas where your terrain heights change over short distances of which you have a lot of here in NY.
If you are worried about tree clutter in the microwave path the 30 meter resolution data available for clutter in Radio Mobile is more than accurate enough provided you don't use the latest vintage version that is available for download. I have found problems with that data set as compared to the next oldest set. I rely on the older version and as it compares to aerial imagery it proves to be more accurate. NED data gets you the actual ground elevations you should use for path and RF studies. If you use SRTM the ground elevation is putting your transmitter at a ground elevation that is inflated by the height of the clutter if there is any in the area. This also creates a false path study because areas of clutter along the path that have clutter are also raised showing as hard terrain which is not the case especially as tools calculate the Fresnel Zone clearances. Use the NED data sets and for clutter use clutter files and adjust the heights of the clutter classes for the typical values in your area. Typical tree clutter in this part of NY are 75 to 80 feet in height. Thank You, Brian Webster www.wirelessmapping.com www.Broadband-Mapping.com -----Original Message----- From: Af [mailto:af-boun...@afmug.com] On Behalf Of Adam Moffett Sent: Wednesday, July 22, 2015 11:31 AM To: af@afmug.com Subject: Re: [AFMUG] NED vs SRTM Does any of the RF modeling software automatically compare NED with SRTM to extrapolate clutter height? It seems like it would be a straightforward feature to add. On 7/22/2015 8:35 AM, Hardy, Tim wrote: > Another point to remember: > > The end points of an SRTM profile sometimes need to be adjusted downwards (or > at the very least, verified) to account for the morphology above terrain that > may have been included in the SRTM data. > > -----Original Message----- > From: Af [mailto:af-boun...@afmug.com] On Behalf Of Brian Webster > Sent: Tuesday, July 21, 2015 11:22 PM > To: af@afmug.com > Subject: Re: [AFMUG] NED vs SRTM > > NED is the national elevation dataset and is based on actual ground elevation > data and digitized topo maps. > > SRTM is a combination of RADAR and LIDAR data from the shuttle missions. They > do not reflect actual ground elevation, they are a processed echo return of > whatever radar and LIDAR returns that were received on the shuttle. > > The source of the data should also be known as to the resolution. SRTM data > was published at 30 meter resolution meaning the hard data points are spaced > at 30 meter intervals. NED data is available in various resolutions the most > common nationwide being 1/3 arc second or 10 meter resolution or actual data > points every 10 meters. This produces a much more accurate terrain model > which in turn creates much more accurate RF propagation information. > > I have attached a real rough paper with images that illustrate the various > differences in terrain data and resolution. > > > > Thank You, > Brian Webster > www.wirelessmapping.com > www.Broadband-Mapping.com > > > -----Original Message----- > From: Af [mailto:af-boun...@afmug.com] On Behalf Of Adam Moffett > Sent: Tuesday, July 21, 2015 6:19 PM > To: af@afmug.com > Subject: [AFMUG] NED vs SRTM > > If there's a 5 meter discrepancy between USGS National Elevation Data and the > Shuttle Radar Topography Mission, who would you trust? Would you bet $500 on > it? > > In this case, with NED I'll have LOS on a backhaul, and with SRTM I'm > in the woods. I'm under the impression that SRTM sometimes gets you > treetops rather than the ground. If I'm wrong I have alternate paths, > but I'd have to pay for a new PCN....so it really is a $500 bet. :) >
