topic Re: Rainfall erosivity map preparation in Mapping Questions

Rainfall erosivity map preparation

Kofi_Lartey — Sun, 21 Feb 2016 17:47:00 GMT

I am trying to develop rainfall erosivity map over a study area using 30yrs period rainfall data. i have determined the R for each year, rainfall erosivity for each year, how do i use this to prepare a map showing the rainfall erosivity for the entire study area.

Re: Rainfall erosivity map preparation

DanPatterson_Retired — Sun, 21 Feb 2016 21:58:16 GMT

basic mapping, if erosivity is associated with a specific surface type. In order to map an attribute you need to associate it with space via a surrogate attribute is a spatial attribute isn't used in its determination

Re: Rainfall erosivity map preparation

Kofi_Lartey — Mon, 22 Feb 2016 12:11:18 GMT

the data was collected from a single station, i now have the location of the station and the elevation, the challenge is how to extrapolate this to cover an area of about 15000 ha. I have looked at the attribute s of a countrywide rainfall map and tried to see how i could use a surrogate attribute (if i am able to clip the study area out), but i dont see the possibility.

Re: Rainfall erosivity map preparation

DanPatterson_Retired — Mon, 22 Feb 2016 12:42:15 GMT

You can't extrapolate or interpolate...you can only assign the attributes to a boundary file covering your study area, perhaps by joining the attributes of your table to the table of the boundary file.

Re: Rainfall erosivity map preparation

Kofi_Lartey — Mon, 22 Feb 2016 18:01:49 GMT

Or mine, this is the sort of missing link i have been looking for, i will try it and get back to you

Re: Rainfall erosivity map preparation

Kofi_Lartey — Tue, 23 Feb 2016 10:43:08 GMT

i saved the excel file as a csv (ms-dos) document and opened it in arcgis.it had only two columns, the years and the R values. When i tried to join with a map of the watershed of the site all the figures were converted to <null> sign. I tried with a boundary shape file with same results. I even tried joining the tables in reverse order but the same results, and i think there is something very wrong with the result. Any suggestions?

Re: Rainfall erosivity map preparation

DanPatterson_Retired — Tue, 23 Feb 2016 10:58:31 GMT

You can join text fields to text fields and number fields to number fields... however, in either case the fields that are used in the join need to contain the same data, so if you want to join based on some class, both files need to contain the same classes

Perhaps some visuals from the help files will help Joining attributes in one table to another—Help | ArcGIS for Desktop

Re: Rainfall erosivity map preparation

Kofi_Lartey — Tue, 05 Apr 2016 20:42:59 GMT

Dan Patterson, this is the latest on the rainfall erosivity map.

I now have data from 6 different stations within the study site, which i used to derive rainfall erosivity values for between 40-11years period. I used the addxy coordinates tool to locate the raingages in a map and using the feature to feature tool in arcmap converted the processed data in excel into a feature. I tried using the Spline approach to interpolate the erosivity values to cover the entire site, i also used krigging but what i am getting is not what i want. How do i improve it. I am attaching two maps for clarification

Re: Rainfall erosivity map preparation

Kofi_Lartey — Tue, 05 Apr 2016 20:44:09 GMT

The first map shows location of raingages and the second results of spline, but which covers only limited part of site. How do i extent it to cover rest of site

Re: Rainfall erosivity map preparation

DanPatterson_Retired — Tue, 05 Apr 2016 22:32:19 GMT

Your extent was set by default, to the extent of the point layer. In order to get a larger extent, you have to set the extent properties in the Environments tab of the tool you are running. That will solve that problem. I will caution, however, that although you may have a minimum number of data points, there are vast areas for which there are no observations and the distribution could be described as poor. I would not put much faith in the pattern that you observe. It may look 'nice' when compared to simple point map, but the interpolated values could at best be described as potentially valid to completely wrong. What you have now, goes well beyond the mechanics of performing an interpolation, into the realm of whether you should be doing one in the first place. That decision resides with you.

Re: Rainfall erosivity map preparation

Kofi_Lartey — Wed, 06 Apr 2016 18:54:28 GMT

@ Dan, i agree with you and this is noted as one of the limitations of the map. I changed the extent in the environment setting and it worked. What i have is a rainfall erosivity map which will be used together with other maps to show a trend. The import of this is what can be done if data is available. Very grateful to all your useful and insightful comments.