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I'm using Arc 9.3.1 (Arc Editor).

In edit mode I've created a simple field calculation. For example's sake let's make it [Field3] = [Field1] + [Field2]

How do I 'keep' the calculation so Field3 will update as I change the values or add values in Field1 and Field2.

I have to keep running the calculation everytime I add data. Cannot it not update automatically?

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I don't think this is possible with the field calculator.

If using a SQL Server or Oracle geodatabase, another alternative is to create a spatial view. In this scenario, Field3 would not be in your original feature class but calculated dynamically in your view.

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I think that you may be best creating a model in ArcToolbox specific for this dataset so when you update the dataset you just run the model.

If you haven't used model builder before then this introduction may help:

...and there is a tutorial here:

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"I have to keep running the calculation every time I add data. Cannot it not update automatically?" Yes Manually.

If have the VB skills you can semi-automate it Create a custom tool in Visual Basic Editor in ArcMap.Each time you want to calculate values for a field, you can just press on this custom tool icon button.

Fastest Method: The easiest and simplest method is EC_Plus:

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If you've got some sql skills and a sde database, you could set up a spatial view of your data with a new field added that is equal to [Field1] + [Field2].

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