Hello List: Was wondering if anyone has experienced anything similar to the following. Let me state initially this is probably my lack of understanding not a software problem. I have an .ERS file that is several USGS quads in width and height. The pixels reflect elevation heights and the regions within the .ERS file reflect the 7.5' neatlines of the quads.
When building a USGS DEM for instance, it is important to ensure that the center of each pixel is at a UTM coordinate that is evenly divisible by the cell or pixel resolution (i.e. 30 or 10 meters). Typically after I generate the expansive, multiquad DEM, I choose a pixel and using the geoposition wizard, I assign that pixel (i.e. 1275.5, 2345.5) a known point UTM position (i.e. 375650, 4625010). This effectively shifts the grid a fraction of a pixel and assures my pixel centroids are in the correct UTM position relative to being evenly divisible by the resolution. I then burn in my regions for the individual quad neatlines. I use a formula to turn all pixels outside the region to null and leave all pixels inside the region alone and save as a new algorithm. I crop the edges of the algorithm window and save to a new, smaller .ALG. When I load that new .ALG and overlay the neatline polygon from which I built the region, everything fits perfectly. If I use the geoposition tools in the VIEW option and key in any pixel (i.e. 1275.5, 2345.5) I get a UTM with all zeros to the right of the decimal point. Perfect. However if when I use my formula and save as a new .ERS my grid shifts slightly and my UTM values at pixel centroids no longer are evenly divisible by the resolution. They have lost that "registration point" information. Obviously, I should just save individual quads as .ALGs. Well here is the rub. When I export the .ALG or even reposition the .ERS, and then export it to .XYZ, I get extra (non-null) pixels on some quads, typically along the north and east edges. This is random and intermittent and typically only one extra pixel at the end of each column. If I have 15 regions of adjacent quads in an .ERS and extract all to .ALG and then go directly to export XYZ, 30% - 50% will have extra pixels. If I re-introduce the same regions to the same .ERS file and convert to .ALG and export to XYZ, several will now not have extra pixels. I simply keep doing this until none have extra pixels. Am I missing something here? I never re-shift the original .ERS file and all the neatlines are exactly correct and fit perfectly side by side. Does ERM change position of the data ever so slightly upon export to .XYZ? Perhaps I am not understanding subtle issues with projected data and ERM. I run into this problem on every WINNT and WIN2000 machine I have used running ERM 6.0 - 6.21. Any thoughts would be very helpful. Greg ----------------------------------------------------------- To make changes to your subscription, please visit our website, http://www.ermapper.com/technicl/ermapperl/index.htm
