topic Log transformation in ArcGIS 10 in Spatial Statistics Questions

Log transformation in ArcGIS 10

EifLeffie — Sat, 13 Aug 2011 05:58:04 GMT

Dear experts and valued members,

I am doing OLS and cant come up with a better model. I found out that some of my variables need transformation however, I can't figure out the right transformation of my explanatory variables as I am not that well verse in stat. I have attached the screenshot of what I did in ArcGIS 10.

Explanatory variables: DEM and MAR (mean annual rain)
Log transformed Field: demtra and lograin

Formula: Demtra = Log ([DEM])
Formula: MAR = = Log ([MAR])

Demtra = Log ([DEM]) (result is highlighted in light blue). I did the same calculation in Excel, when I used the Log function in excel I got different results. I tried LN (natural logarithm) in Excel and got the same result as Log ([DEM]) in ArcGIS. I am wondering if I am doing the right transformation using the above formula when there is a need to transform the data to logarithm. The DEM variable is skewed to the right as such I am doing log transformation.

I did the same transformation for MAR (mean annual rainfall) and the Lograin (last column) resulted to single values though I indicated a precision of 2 when I added a new field.

I would appreciate any assistance on this. Thank you so much for your valuable comments and time in helping out.

Cheers,
Eiffel8

Re: Log transformation in ArcGIS 10

LaurenRosenshein — Mon, 15 Aug 2011 21:26:18 GMT

Hi Eif,

It looks like you are using the right Log function in ArcGIS. The "Log" function using VB is the same as the "Ln" function in Excel...both use the Natural Log. The "Log" function in excel actually defaults to a log with a base of 10, so that is where the difference you're seeing is coming from. Sorry for the confusion there!

As for the precision issue that you're having, it looks like the field that you are using to calculate the log is an integer field (either short integer or long integer). You will need to create new fields and use either Double or Float, which will then give you the precision that you're looking for.

Re: Log transformation in ArcGIS 10

EifLeffie — Thu, 18 Aug 2011 06:49:15 GMT

Thanks much Lauren for the help!..so helpful indeed.
Cheers,
Eif