topic How to Log-Transform the data? in ArcGIS GeoStatistical Analyst Questions

How to Log-Transform the data?

ROMANFILOZOF — Wed, 10 Apr 2013 13:46:22 GMT

Hi everybody,

Have some troubles with my data while trying to use OLS and then The Exploratory regression. The report interpretations tell that I should try to transform some skewed variables. I've seen that advise a few times in Help too. But I haven't found the tool that permit to do that. As I understand - I need a log transformation (Histogram tool showed better result with it). So how should I "log-transform" my data. Geostatistical Wisard creates some rasters as I understand, and I need to get new (log-transformed) values in the attribute table for using them letter in OLS again.

Could anybody help with some advise?

Re: How to Log-Transform the data?

EricKrause — Wed, 10 Apr 2013 14:48:07 GMT

You'll need to make a new field in your feature class and use the field calculator. The field calculator has a built-in Log() function.

Re: How to Log-Transform the data?

ROMANFILOZOF — Thu, 11 Apr 2013 09:00:05 GMT

Thanx, Eric,

I tried to do so, but a strange thing happens:
When I use the Histogram tool with the original field applying log transformation inside the tool - I see one diagram, but if I calculate new field and use the Histogram tool without any transformation - the diagram is different. As I've understood from Help, the reason is in the algorythm used by log transformation inside the Histogram tool - it uses kriging log transformation meanwhile as the build-in Log () function is pure Log.

So there's no another way to get the same result as shown in Histogram tool?

Re: How to Log-Transform the data?

EricKrause — Thu, 11 Apr 2013 14:19:53 GMT

I just tested, and I get the same graphs whether I take the logarithm in the histogram tool or make a new field and calculate the logarithm. I suspect you've made a mistake somewhere, but I don't know where it could be. Try it again and see if it works this time.

The Logarithm function in the histogram isn't doing anything special. It is just taking the natural logarithm, exactly the same as the Log() function in the field calculator.