Computers have specific ways of storing numbers. How the numbers are stored determines the minimum and maximum values and whether the number can have fractional values.
| Name | Number of Bits | Number of Bytes | Minimum Value | Maximum Value | Number of Values | Significant Digits |
| Bit | 1 | 1/8 | 0 | 1 | 2 (21) | <1 |
| Chew | 2 | 1/4 | 0 | 3 | 4 (22) | <1 |
| Nibble | 4 | 1/2 | 0 | 15 | 16 (24) | >1 |
| Unsigned Byte | 8 | 1 | 0 | 255 | 256 (28) | >2 |
| Signed Byte (aka chars) |
8 | 1 | -128 | 127 | 256 (28) | >2 |
| Unsigned Short | 16 | 2 | 0 | 65535 | 65536 (216 or 64k) | >4 |
| Signed Short | 16 | 2 | -32768 | 32767 | 65536 (216 or 64k) | >4 |
| Unsigned Integer (Int) | 32 | 4 | 0 | 4,294,967,295 | 4,294,967,295 (232 or 4Gig) | >9 |
| Signed Integer | 32 | 4 | -2,147,483,648 | 2,147,483,647 | 4,294,967,295 (232 or 4Gig) | >9 |
| Long Integer (always signed) |
64 | 8 | A big negative number | A big positive number | 264 | >19 |
| Float (always signed) |
32 | 4 | ~ -1040 | ~ 1040 | 232 | ~7 |
| Double (always signed) |
64 | 8 | ~ -10300 | ~ 10300 | 264 | ~15 |
The available types of numbers in a modern computer.
In the table there are Integer values (Bit, Chew, Nibble, Byte, Short, and Integer). These values can only contain whole numbers. There are also Floating Point values (Float and Double). These values can have fractional values associated with the number. This is a very important distinction and you need to select the correct format for your numbers.
In addition to numbers, computers can store dates and strings. String are stored as sequences of characters. This includes the text you are currently reading. Each letter is a "character" in a string.
Dates are stored with their year, month, day, hour, minute, and second. You may see dates in two common formats:
When you create an attribute, you have to select a data type. The list of available types is shown below for ArcGIS Attributes, ArcGIS GRIDS (and other rasters in ArcGIS), and in Geodatabases. The recommended types are in bold.
| Name | ArcGIS Attributes | ArcGIS Rasters | Geodatabase |
| Unsigned byte | Unsigned 8 bit | ||
| Signed byte | Signed 8 bit | ||
| Unsigned short | Unsigned 16 bit | ||
| Signed short | Short Integer | Signed 16 bit | Short Integer |
| Unsigned Integer | Unsigned 32 bit | ||
| Signed Integer | Signed 32 bit | Long Integer | |
| Long | Long Integer | ||
| Float | Float | Floating-point 32 bit | Single-precision floating point |
| Double | Double | Double-precision floating point | |
| String | Text | Text | |
| Date | Date | Date |
| ArcGIS Name | Name | Data Type | Description |
|---|---|---|---|
| Name | Name | String | Could be the name of a town, state, or college |
| Species | Species name | String | The scientific name of a species |
| NumPeople | Number of People | Long Integer | Number of people in the feature |
| Area | Area | Double | The area of a state, county or other polygonal region |
| Length | Length | Double | The length of a stream or road |
| Easting | Easting | Double | X coordinate value |
| Temp | Temperature | Double | Temperature (units should be in the metadata) |
| NumTrees | Number of Trees | Long Integer | Number of trees in the feature |
| Height | Height | Double | Height of a tree (units in metadata) |
| DateColl | Date Collected | Date | Date the data was collected |
| CollName | Collector Name | String | Name of the person who did the collecting |
First, note that I have to use 10 characters or less for the name of the attribute. This is a limitation of the ArcGIS software. Also, it is best to not use any punctation except the underscore ("_") character.
Note that I have used Double for all floating point types and "Long Integer" for all integer types. This will ensure that you do not run out of precision.
Note: Selecting the correct data type for attributes is critical, something you'll be doing regularly, and you'll be tested on it.
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