[ai-geostats] A question on lag class and lag distance
Dear all, I have a question on the selection of the lag classes and lag distance when computing experimental semi-variogram. It has been suggested that the average semivariance can be done in an increment with an arbitrary distance tolerance (e.g. 300 m ± 150 m) taking into consideration of the number of data pairs per lag class. However, the lag class and lag distance are set a bit arbitrarily, and can influence the resulting variogram. My question is: 1. How to select the lag class and lag distance in order to obtain a more reasonable experimental variogram? 2. Is it reasonable to use an uneven set of lag (e.g. the lag increments are: 0-2.5m, 2.5-5.0m, 5.0-12.0m, 12.0-19.5m, 19.5-27.0m, 27.0-30.0m, 30.0-40m, 40-50m etc.) if a more stable variogram can be obtained? I should really appreciate anyone’s reply! Thanks a lot ahead! XiuhDo You Yahoo!? 150万曲MP3疯狂搜，带您闯入音乐殿堂美女明星应有尽有，搜遍美图、艳图和酷图 1G就是1000兆，雅虎电邮自助扩容！* By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
Re: [ai-geostats] A question on lag class and lag distance
xhy your questions are long-standing and as yet unanswered in general. > 1. How to select the lag class and lag distance in > order to obtain a more reasonable experimental > variogram? I always think of it as focussing a camera. Believe there is a pattern in your data and our task is to balance 'width of interval' versus 'number of pairs in interval' to get the clearest picture. One of the things I have found most useful with irregularly spaced data is a 'nearest neighbour' analysis. Take each sample and find the closest one to it. Record the distance. Repeat for all samples. This process takes twice as long as calculating the semi-variogram but gives you an idea of the 'natural' or model spacing between your samples. This can be used to guide your choice of interval. Check out our free tutorial downloads at http://geoecosse.bizland.com/softwares > 2. Is it reasonable to use an uneven set of lag > (e.g. the lag increments are: 0-2.5m, 2.5-5.0m, > 5.0-12.0m, 12.0-19.5m, 19.5-27.0m, 27.0-30.0m, > 30.0-40m, 40-50m etc.) if a more stable variogram > can be obtained? I am not sure I have ever seen this done, but don't see why not if you plot the point at the centre of gravity of your interval (i.e. average distance of pairs found). Hope this helps Isobel http://geoecosse.bizland.com/books.htm ___ALL-NEW Yahoo! Messenger - all new features - even more fun! http://uk.messenger.yahoo.com * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
