--- Start of Idrisi List message Hi List, I am working with the NDVI dataset for Africa from Clark University, which is provided in Clark Labs Hammer-Aitoff projection. I need to extract values from these data for locations corresponding to points in a vector file. Here is where I run into problems: 1. There appears to be no direct way to perform an attribute query on the raster data, using these values to update the attribute tables associated with my vector point locations. (in other words, for each location in my point layer, find the value in the raster layer I'm interested in and plug that value into a field in the point layer's attribute data.) 2. Because this is an easy task to perform in ArcView, I thought to export the data and do the query there. Here, however, I run into another set of problems: a) I cannot export the data in the Hammer-Aitoff projection - the Idrisi export modules fail for some reason. Why? There is no reason why this shouldn't work, right? b) I can export the data after it has been reprojected into decimal degrees. BUT, I have some new problems here too, mostly related with my ignorance and insecurities about how the PROJECT module does its work: if I accept the new bounding coordinates (min and max x and y), I have to manually recalculate the # of rows and columns in order to avoid distortion in the pixel dimension in other GISs. (Idrisi appears to effectively stretch the grid matrix between the bounding coordinates, whereas ArcView, for example, takes an origin [lower left corner] and calculates position from there using a uniform pixel dimension - this means that some dd data that displays properly in Idrisi, is very distorted when displayed in ArcView.) By doing the manual recalculation of row and column #s, I can get my data to appear normal and properly positioned, BUT... is there a way to describe/quantify the distortion produced by doing this (which is also going to vary across the geographic extent of my data) -? I feel uneasy about this. Ok, here is the final problem: 3. A work around to problem 2a above is to import the idrisi raster files directly into ArcView using ArcView extensions. Here, however, my data do not display coincidentally with data in dec.degrees that has been projected within ArcView to Hammer-Aitoff. It seems apparent to me that the "Clark Labs Hammer-Aitoff projection" has some defining parameters that are not part of ArcView's definition of the Hammer-Aitoff projection. But, what are these differences? (In the way of background to this projection issue: the .ref file for the Idrisi data contains the following information: ref. system : Clark Labs Hammer-Aitoff Grid for Africa projection : Hammer Aitoff datum : WGS84 delta WGS84 : 0 0 0 ellipsoid : Mean Sphere major s-ax : 6370977.000 minor s-ax : 6370977.000 origin long : 17 origin lat : 1 origin X : 5000 origin Y : 5000 scale fac : 1.0 units : km parameters : 0 The ArcView dialog for selecting the Hammer-Aitoff projection has only one option for adjustment: specification of a central meridian. I have experimented some, but there appears to be no straightforward offset, like a simple linear shift or something, and I lack enough knowledge of projections to know what I need to do to get my two datasets coincident within ArcView. Has anyone had to do this?) OK, I realize that this bundle of questions deals with quite different issues. They're really only related because of my particular situation (i.e., if I could solve problem 1, I would face problems 2 or 3; etc.). Sincere thanks in advance for any and all help on one or more of these problems. Regards to all, jc --- End of Idrisi List message IDRISI-L is an unmoderated list. As a service to list users, please post a summary of useful responses to your questions. To post a message to IDRISI-L, send it to [EMAIL PROTECTED] To subscribe to the list, send an email from the address to be subscribed to [EMAIL PROTECTED] with "subscribe" (without quotes) as the first line of the message body. To unsubscribe, do the same, only with "unsubscribe" as the first line of the message body. The list is archived at http://www.mail-archive.com/idrisi-l%40clarklabs.org/.
