Problem:  Symbolizing in ArcMap using transparency by field producing inaccurate results

Article ID: 21444 N/A ArcGIS - ArcEditor 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcInfo 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcView 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1, 10 N/A

Description

Inaccurate results when using field-based transparency in ArcMap.

Example: Mapping US_States by POP90_SQMI and using field-based transparency. Illinois and Georgia are symbolized similarly even though the data values are significantly different. Illinois has a population density of 203 persons per square mile, which is interpreted as 3% transparent. Georgia has a population density of 110 persons per square mile, which is interpreted as 10% transparent. Both states appear to have similar population densities on a map produced with these results.

Cause

Transparency by field interprets data values as a percentage transparency. Consequently, it does not deal well with values greater than 100. For values greater than 100, it performs a modulus operation on the value.

Solution or Workaround

Avoid using transparency by field with data that ranges below 0 or above 100. If necessary, add a new field and calculate a re-scaled data range.

2. Determine the minimum and maximum values in the field whose values need to be scaled to a range between 0 and 100 (OldField). Right-click on OldField and select Sort Ascending. Note the minimum, then scroll down to the bottom and note the maximum.
3. Click Options and select Add Field. Add a new Short Integer field. Call this field NewField.
4. Right-click on NewField and select Field Calculator.
5. Enter the following expression, inserting the actual min and max for <OldField Maximum> and <OldField Minimum>:
```(100 * ([OldField] - <OldField Minimum>)) / (<OldField Maximum> - <OldField Minimum>)
```

Be sure to include the negative sign for negative values.
6. Click OK to calculate the values for NewField.

Created: 1/21/2002

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