Hi all I'll preface this with saying that I'm relatively new to ENVI, so I'm probably missing something quite simple. If so, great! And here's my problem: I would like to rectify some aerial photographs, taken from a helicopter [hence some pitch and roll is usually present]. I have all the parameters needed for building an external orientation, since I know the latitude, longitude, altitude, pitch, roll and heading of the camera - everything required for omega/phi/kappa correction. I don't have any ground control points, but in my favour, there's essentially no topography - I can assume that the terrain is flat [I'm looking at sea ice]. Using ENVI's build RPC and orthorectification tools, it looks like it should be possible to rectify the images using camera information and external orientation parameters. However, I'm clearly doing something wrong. My questions, then are: - in the build RPCs dialong, under 'edit exterior orientation parameters', what are the units for XS, YS and ZS? I've assumed that these define the geographic location, but using lat/lon/alt doesn't appear to work. The next couple of issues are quite likely to be related to the first one: - If I attempt to rectify an image using a generated set of RPCs, the output projection section of the orthorectification dialog box shows some odd values. I assume it tries to get the output size from the image and RPC information, but the figures are out by a lot - usually many thousands of pixels bigger than the image really should be. - another problem is that the output projection dialog asks for an upper left corner co ordinate. I can guess at it, but I don't really know it. All I have is the location of the camera, somewhere near the middle of the image [depending on aircraft pitch]. Any hints or useful information will be greatly appreciated. I'm missing something along the way, I think [aside from ground control points ;-) ]. Thanks Adam