GEOG 181 Week 5

Map accuracy

How closely the information on a map matches the true value

Map precision

How exactly the data is being measured

As precision increases, so does accuracy?

False, precision can increase but accuracy can stay the same

Operational errors

Errors associated with the collection, management, and processing of geospatial data

Systemic error

Error tends to shift all measurements in a certain way, such that the mean value of an error may be predicted

Random error

Error occurs in ways that cannot be predicted

Five categories of uncertainty

1. Lineage

2. Positional accuracy

3. Attribute accuracy

4. Logical consistency

5. Completeness

Data lineage

Where data comes from and where it was sourced. Data sources have different standards of accuracy

Positional accuracy

The horizontal and vertical accuracy of the data

Attribute accuracy

The accuracy of features found at a particular location

The correct identification of a particular soil type at a specific location is an example of

Attribute accuracy

Logical consistency

Relationships that exist between features on the map

A question like, “are all polygons on the map closed?” is an example of

Logical consistency

Completeness

The inclusion of all relevant features on a map

The proper documentation of all mapping standards used to prepare data is an example of

Completeness

Map scale

The ratio of distance on a map to distance in the real world

What is the only location on a map in which there is zero distortion?

Along the standard point or standard line(s)

Principal scale

The scale of a map at the standard point or line(s)

Scale factor

The ratio of local map scale for a specific location on the map to the principal map scale

Calculating scale factor

\large \text {Scale factor} = {\text {Local Scale} \over \text {Principal Scale}}

At the standard point or line(s), the scale factor is equal to

1

At locations where features are compressed, the scale factor is

Less than 1

At locations where features are stretched, the scale factor is

Greater than 1

North-South scale

One degree of latitude is 111km on the Earth
\text {North-South Scale} = {\text {map distance} \over \text {real world distance}}

East-West scale

We must use \text {length of one degree longitude} = \text {(111 km)*cos(latitude)} to calculate the length of a degree of longitude