Thank you very much to all people who answered my question. It was very helpful. Here�s a summary of the responses. Cheers, Carolina
Dear list members, I have started using geostats only this year and therefore I appologise that my questions are too basic for this discussion list... I am using Variowin 2.2 and GeoEAS 1.2.1 to analise my data, and an early version of Surfer to contour the distribution. However, I've encountered various problems: 1. When I use latitude and longitude in my DAT file, in what units is the variogram x-axis (h)? 2. Surfer doesnt understand the grid file created by GeoEAS KRIGE. How do I transfer my results to surfer (until now, I've used the estimated values from cross validation to have surfer generate a grid file)? 3. My variogram shows completely different behaviour from distance A to B, than for distance B to C. I've tried fitting various nested models, but I havent been able to come out with a good fit for the whole variogram. Is it possible to analise A-B data separately from B-C data? Hi Carolina, I'm new at this too, so have an aswer to your first question only. :) Basically, you can't use latitude and longitude to indicate the location of your samples in a geostatistical analysis because the distance between locations is not euclidean. When you calculate variograms, you are assuming that you are on a plane X-Y coordinate system. Lat and log account for the curvature of the earth. So unless you are dealing with very short distances you should first convert your lat and long locations into UTM coordinates (or other plane projection). I use a program called geotrans to convert lat and long to UTM, you can download it at http://164.214.2.59/GandG/geotrans/geotrans.html Once you have your locations in UTM, the variogram distance are measured in meters. Julian Julian M. Burgos University of Washington School of Aquatic and Fishery Sciences 1122 NE Boat St. (Box 355020) Seattle, WA 98195-5020 1- You need to transform your latitude longitude geographical data into UTM coordinates (or any equivalent transformation) because you need to work in metres or km (or cm!) > 2. Surfer doesnt understand the grid file created by GeoEAS KRIGE. How do I > transfer my results to surfer (until now, I've used the estimated values from > cross validation to have surfer generate a grid file)? If you want to krige witn Geo-Eas you have to use its CONREC program to have your map. Otherwise, if you like to use Surfer, you just need to krige there, using its own grid utility. > 3. My variogram shows completely different behaviour from distance A to B, than > for distance B to C. I've tried fitting various nested models, but I havent been > able to come out with a good fit for the whole variogram. Is it possible to > analise A-B data separately from B-C data? What are A,B,C? any kind of different variables, or just some different locations? If there are just different points you need to know the ir reliability before doing any assunption on them. If there are different variables, you do not need to suppose they behave in the same way. I think there is something wrong with your variogram. If variables are quite the same for small distances (flat variogram) they cannot start being different from one each other at large distances. Maybe you have chosen a bad lag (h), say a lag too short. It is very important for you to know the precision of your data (it doesnt make any sense to say temperature is 10.5 if your thermometer measures degrees!!) Hope this helps, Carme Carolina > >1. When I use latitude and longitude in my DAT file, in what units is the >variogram x-axis (h)? > Most geostatistics programs are going to treat the coordinates as though they are "x", "y", i.e., rectangular. If you are using Lat, Long these are not exactly rectangular coordinates. That is, one minute of Lat is one nautical mile but in general one minute of Long is not one nautical mile, the distortion can be very large at high latitudes. You need to change to UTM coordinates instead. As a further comment, most packages will only accept numbers in decimal form so if you have Lat, Long in degrees, minutes and seconds the program will either give you an error message or completely mis-interpret the entries. I let my students use Lat , Long in decimal form in my geostatistics class but ONLY in class and I warn them that there will be a distortion. The only reason I do this is that they are learning about how to do geostatistics and not using the end results for anything outside of the class. > >2. Surfer doesnt understand the grid file created by GeoEAS KRIGE. How do I >transfer my results to surfer (until now, I've used the estimated values from >cross validation to have surfer generate a grid file)? > I don't use Surfer so I can't completely answer your question. I assume by "grid file" you mean the output file from KRIGE. This is an ASCII file and easily edited in any text editor. The format is exactly the same as the input file (different column names of course). If you only estimated one variable then there will be four columns; two for coordinates, one for the kriged estimate and one for the kriging standard deviation. The header identifies the columns. You might have to strip off the header for use in Surfer. >3. My variogram shows completely different behaviour from distance A to B, than >for distance B to C. I've tried fitting various nested models, but I havent been >able to come out with a good fit for the whole variogram. Is it possible to >analise A-B data separately from B-C data? > I assume that A, B, C are points?? If so, your statement doesn't quite make sense. The sample variogram is not defined or dependent on a line segment. The plotted values of the sample variogram (or the computed values of the model variogram) are determined by lag distances. In that case, there is no reason to expect that the value for two different lag distances would be the same. Perhaps you are referring to different directions? In that case you would be describing an anisotropy, this would show up (assuming that you are using PREVAR and VARIO or using VARIOWIN) when you compute and plot directional sample variograms. For a geometric anisotropy, this would show as a change in the range of the variogram with respect to distance. Finally is it possible you are asking whether the region of interest can be split into two separate parts?? This is certainly possible, you would have to split the data set into two different parts estimate and model the variograms separately. Krige the two subregions separately using the respective variograms and data sets. It is likely that the grids (generated by the software) would overlap and you would have to check to see which grid nodes should be associated with which subregion and use the appropriate kriged value. If you were going to do this, you would want to ask if there is some property that changes between the regions to justify/motivate splitting the region into two parts. Donald E. Myers http://www.u.arizona.edu/~donaldm Carolina Garcia Imhof Marine Mammal Research Group Marine Science Department 310 Castle Street, PO Box 56 University of Otago Dunedin, New Zealand Fax: 64 3 479 8336 Phone: 64 3 479 5476 e-mail: [EMAIL PROTECTED] [EMAIL PROTECTED] -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and "unsubscribe ai-geostats" followed by "end" on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
