Attribute Types

Key Points

The data type you select for storing information in a computer limits what you can do with it.

Types of Data

There are two general types of data: Qualitative and Quantitative. Qualitative data is nominal, meaning types, but no specific order to them. Quantitative data is either Ordinal (ordered sets of classifications), Interval (values along non-arbitrary scale), Ratio (Interval data with an inherent 0).

An example of Qualitative Data is:

Nominal - Unordered categories
- Nominal data are in categories that do not have an inherent order
- Examples include:
  - Land cover types include: forest, grassland, water, rock, urban
  - Species
  - Soil types

Some examples of Quantitative Data are:

Ordinal - Ordered Categories
- Ordinal data are categories but they have an inherent order
- This includes:
  - Fire danger: low, medium, high
  - Risk of flooding: low, medium, high
Interval - Continuous data without an inherent 0 value
- Internal data are continuous values (not categorical) but there is no definite 0 value.
- Examples include:
  - Temperature
Ratio - Continuous data with an inherent 0 value
- Ratio is similar to Interval, except that it has a meaningful "0" value (i.e. a population density of 0 means no people)
- Examples include:
  - Precipitation
  - Population density
  - Elevation
  - Hieght
  - Number of trees, people, flowers, etc.
The reason these are important is because they deterine the types of calulations you can perform on data:

Can compute:	Nominal	Ordinal	Interval	Ratio
Counts, and frequencies (i.e. histograms)	✔	✔	✔	✔
Median and percentiles		✔	✔	✔
Add and subtract values			✔	✔
Mean, standard deviation, standard error of the mean			✔	✔
Ratio or coefficient of variation				✔

Note: these definitions will appear from time to time but you will not use them on a regular basis

Cyclic - Some values can also repeat over and over. Cyclic values can be ordinal or interval.

Examples include:
- Time of day, day of the year
- Longitude and latitude

Data Types

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 (2¹⁶or 64k)	>4
Signed Short	16	2	-32768	32767	65536 (2¹⁶or 64k)	>4
Unsigned Integer (Int)	32	4	0	4,294,967,295	4,294,967,295 (2³²or 4Gig)	>9
Signed Integer	32	4	-2,147,483,648	2,147,483,647	4,294,967,295 (2³²or 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.

Integer and Floating Point

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.

Dates and Strings

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:

US: MM/DD/YYYY - the MM is number of the month (starting at 1), the DD is the day of the month (starting at 1), and the YYYY is the year.
European: DD/MM/YYYY
Database: YYYY-MM-DD HH:MM:SS - the HH is the hour in 24-hour format, the MM is the minutes, and SS the seconds.

Attributes in ArcGIS

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

Examples of Attributes:

ArcGIS Name	Type of Data	Name	Data Type	Description
Name	Nominal	Name	String	Could be the name of a town, state, or college
Species	Nominal	Species name	String	The scientific name of a species
NumPeople	Ratio	Number of People	Long Integer	Number of people in the feature
Area	Ratio	Area	Double	The area of a state, county or other polygonal region
Length	Ratio	Length	Double	The length of a stream or road
Easting	Interval	Easting	Double	X coordinate value
Temp	Interval	Temperature	Double	Temperature (units should be in the metadata)
NumTrees	Ratio	Number of Trees	Long Integer	Number of trees in the feature
Height	Ratio	Height	Double	Height of a tree (units in metadata)
DateColl	Interval	Date Collected	Date	Date the data was collected
CollName	Nominal	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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