topic Re: Symbology on null value in Data Management Questions

Symbology on null value

JimO_Leary — Sat, 26 May 2012 18:59:23 GMT

I am trying to symbolize a layer on Quantities -> Natural Breaks -> Jenks. There are some null values in the column that I'm using for the symbolization, and that causes those features to disappear from the map when symbolized. Is there a way that I can set a default value for the column so that this will not happen?

Thanks

Re: Symbology on null value

RosieBell — Mon, 28 May 2012 03:48:38 GMT

I am trying to symbolize a layer on Quantities -> Natural Breaks -> Jenks. There are some null values in the column that I'm using for the symbolization, and that causes those features to disappear from the map when symbolized. Is there a way that I can set a default value for the column so that this will not happen?

Thanks

There might be a simpler method than this - but you could add a field, then use field calculator to fill it with the values from the old field. Then use Select by Attribute to select all the rows with a value of NULL, then use Field Calculator again to replace this null value with a different value.

Re: Symbology on null value

JimO_Leary — Tue, 29 May 2012 13:32:45 GMT

I did find one method. I exported the data to a shapefile, upon which the null values became zeros. Then I imported the shapefile and used it as the source for the theme. Now the formerly null values at least showed up as zeros. I added another break in the Jenks that Arc provided, specifying zero.

Re: Symbology on null value

JonathanQuinn — Tue, 29 May 2012 14:49:17 GMT

Another method you can use in the future is the Select By Attribute function to select all records in that field that are null, then the Field Calculator to calculate a 0 for those records; if you have records selected in a field, the field calculator will only run on those records.

Re: Symbology on null value

SarahHolmes — Mon, 16 Sep 2013 12:04:06 GMT

There might be a simpler method than this - but you could add a field, then use field calculator to fill it with the values from the old field. Then use Select by Attribute to select all the rows with a value of NULL, then use Field Calculator again to replace this null value with a different value.

Yes, I tried that too. But because you want to symbolize Quantities, you need a number field and the only way to use Field Calculator is with a Text field so unfortunately this won"t work.

Re: How to Symbolize NULL values separately then Zero values

SarahHolmes — Mon, 16 Sep 2013 12:07:20 GMT

Okay I have a similar problem now only I have some values being zero and some without values at all (NULL) and I need to distinguish between the two because a country with a data value of 0 is different than "No Data". What do I do!?

Re: Symbology on null value

RichardFairhurst — Mon, 16 Sep 2013 12:08:51 GMT

Yes, I tried that too. But because you want to symbolize Quantities, you need a number field and the only way to use Field Calculator is with a Text field so unfortunately this won"t work.

Field calculator works with number fields perfectly fine. Selecting Null values and overwriting them with the Field Calculator is no different between a text field and a number field. Why would you think otherwise?

Re: Symbology on null value

SarahHolmes — Mon, 16 Sep 2013 12:11:49 GMT

:S Well it isn't working for me. Every time I try to use Field Calculator on a number field it is grayed out. Why would that be?

Re: Symbology on null value

RichardFairhurst — Mon, 16 Sep 2013 12:20:56 GMT

:S Well it isn't working for me. Every time I try to use Field Calculator on a number field it is grayed out. Why would that be?

No idea without any information on what version of ArcMap you are using, what database you are using, if you have added any joins to your data, etc. Based I what I know of your set up all I can say is it works for me and definitely should work for you if you are working with normal unjoined geodatabase or shapefile data.

Re: Symbology on null value

SarahHolmes — Mon, 16 Sep 2013 12:40:42 GMT

I am using ArcGIS 10.1. I work in the Maps, Data & GIS Library at a University and a colleague and I were experimenting with what data is available to students as we have a cartography class in which each student has to choose a topic and study area and make a map. We wanted to know what boundary files were available on a global scale and were testing the "country.sdc" layer from ../My Document/ArcGIS/Packages/World_Countries/commondata/data0/ which I know to be some sort of data from Esri. It loads almost like a CAD file. Perhaps this is the problem? We have then joined that file to some data from the Human Development Reports.

Re: Symbology on null value

TimWitt — Mon, 16 Sep 2013 12:44:09 GMT

Sarah,

you can't calculate fields that have been joined to data. You would need to do the calculate before you join it to data. Maybe that is your problem? Remove the join and then you should be able to calculate.

Re: Symbology on null value

SarahHolmes — Mon, 16 Sep 2013 12:46:24 GMT

But the data is IN the join. The data is in the table I am joining to. So that doesn't work.

Re: Symbology on null value

SarahHolmes — Mon, 16 Sep 2013 13:03:32 GMT

Okay I found a way to make it work... although I still think there should be an easier way. I created a new field and used Field Calculator to define the values from the column I was working with. However, if I just did them all at once, all the <NULL> values became zero which I couldn't have but I also couldn't define them as No Data because it's a number field and won't accept text. So first I used FC to make all the <NULL> values a value outside my existing range of values, then calculate the rest of the values normally. Then I classified the values to that the new, outside value was in a class on its own and I changed the label to "No Data". I think this is harder than it should be but at least it works I suppose. However my issue now is that, because I had to use a higher value than the rest, "No Data" appears on TOP of the rest of the values in the legend where I think it should be at the bottom, under 0.

Re: Symbology on null value

Zeke — Mon, 16 Sep 2013 13:11:55 GMT

You can change the order symbols display in the legend using the up/down arrows on the symbology tab.

I don't understand what the problem is with calculating the field before the join (couldn't you calculate it in the table first, using Excel or similar?), but in any case, once you've joined the table you can then export the whole thing to a new feature class and use that. FC will work on that.

Re: Symbology on null value

RichardFairhurst — Mon, 16 Sep 2013 13:25:40 GMT

Okay I found a way to make it work... although I still think there should be an easier way. I created a new field and used Field Calculator to define the values from the column I was working with. However, if I just did them all at once, all the <NULL> values became zero which I couldn't have but I also couldn't define them as No Data because it's a number field and won't accept text. So first I used FC to make all the <NULL> values a value outside my existing range of values, then calculate the rest of the values normally. Then I classified the values to that the new, outside value was in a class on its own and I changed the label to "No Data". I think this is harder than it should be but at least it works I suppose. However my issue now is that, because I had to use a higher value than the rest, "No Data" appears on TOP of the rest of the values in the legend where I think it should be at the bottom, under 0.

On your earlier issue, I suspected you had a join involved. Null values are created 2 ways with joins. First, if no record in the joined table matches the table it is joined to, then the entire record is Null. This is the only way joined data can appear when it has no matching value and the only way to overcome it is to add a record to the joined table that matches the join field value, or remove those records from the primary table. The second occurs when a record match between tables exists in the join field, but the field you are using to symbolize has a Null value. In this case you must directly open the table that you intend to make a join table outside of the join all by itself. Then you can calculate the values in the original table and when the table is joined the values will not be null anymore. Bottom line is that in both cases you have to edit the original table you intend to join to both ensure a complete record match and complete field data before creating a join.

Your solution is fine and actually not that difficult or involved compared to most problem solving in GIS. You got off easy. Get used to the feeling that there should be an easier way and finding out there isn't with this software. That is the norm.

Anyway, as far as the number sorting, what is wrong with using -1 (or some other negative number) as a value for the Nulls? That would make the no- data values appear at the bottom of your list.

Re: Symbology on null value

SarahHolmes — Mon, 16 Sep 2013 13:44:23 GMT

Richard,

I am aware of how and way NULL values appear and that what you suggested is how to removed them. But technically I don't want to removed them, I want to map them. And I can't alter the attribute table to make the NULLS zero because I have zero values which I need to keep separate. So basically what everyone is saying is that you cannot make <Null> values display in the symbology?
Okay well at least I have my work-around way. Negative values! Brilliant! Thank you; I had not even considered negatives!

Re: Symbology on null value

SarahHolmes — Mon, 16 Sep 2013 13:49:21 GMT

You can change the order symbols display in the legend using the up/down arrows on the symbology tab.

I don't understand what the problem is with calculating the field before the join (couldn't you calculate it in the table first, using Excel or similar?), but in any case, once you've joined the table you can then export the whole thing to a new feature class and use that. FC will work on that.

Greg,

Changing the order in the legend doesn't apply here as I am talking about classifications of one layer.

Okay, I understand now what you meant about calculating before I join and yes that would work but I am stiff miffed as to what I would defined them as because I want them to be No Data (which you can't do in a number field).