Data Standards: Scale, Accuracy, Precision

Question 1

What is scale?

Answer 1

• Scale can refer to several different concepts including grain (spatial resolution), extent (landscape size) & cartographic ratio (map scale).
• Cartographic ratio or map scale reflects how much an object(s) is reduced from its true spatial extent (true landscape size).
• The scale of a map reflects how accurately the location and shape of the features can be depicted on the map or in our GIS database, and how much detail can be presented
• The ratio component of a maps scale tells us how many units of the real world are represented by a smaller unit on the map.
• Scale: The ratio tells us how many units of the real world are represented by a smaller unit on the maps.
• Given
• 1:10,000 map
• 1cm on the map
• 10,000cm (or 100m) on the ground in the real world..

Question 2

What is small scale?

Answer 2

❑ Small area on map
covers large area on the ground
❑ E.g. 1:1million map
1cm on map = 1,000,000cm on the ground (or 1,000m)
❑ Coarse scale

Question 3

What is large scale?

Answer 3

❑ Same sized area covers
small area on the ground
❑ can represent more detail of what is on the ground
❑ E.g. 1:10,000 map
1cm on map = 10,000cm
on the ground (or 100m)
❑ Fine scale - more detail you capture the larger the file size.

Question 4

What do we need to consider when calculating map scale?

Answer 4

• Ground Distance
• Map distance

Question 5

What is scale ratio?

Answer 5

ground distance / map distance

Question 6

What is the grain scale?

Answer 6

in raster data = spatial resolution of an image is the grain ratio.

Question 7

What is spatial resolution?

Answer 7

is the size of each raster grid or pixel making up the larger raster image.
pixel size determines the details of a raster image.

Question 8

What is accuracy?

Answer 8

Accuracy of data is the degree to which data correctly reflects the real world object OR event being described.

how true the meausruement of recorded values is to the true values.

how far the recorded values are from the true value, this calculation gives the true dataset.

Question 9

What is error?

Answer 9

the difference between the measurement of recorded values and the true values.

Question 10

What is accuracy in GIS?

Answer 10

• is how close the recorded location of a spatial feature is to its ground location
• The degree to which information on a map or in a digital database matches true (or accepted) values.
• Accuracy is partially determined by map scale
• Geographical positional accuracy:
• Horizontal (x:y)
• Vertical (z)
• Attribute accuracy and logical consistency

Question 11

What is precision?

Answer 11

• Precision measures how exactly the location is recorded
• How many decimals on your DMS GPS reading
• Production level GPS (5m) or DGPS (< 1cm)
• Precision is the repeatability of measurements i.e. How may times measured.
  high precision does not indicate a high accuracy!! and high accuracy does not imply high precision!!

Beware:
Precisely measured data can still be inaccurate
If you are standing at the wrong spot with your DGPS

Question 12

What level of precision and accuracy does your project require?

Answer 12

• High precision and accuracy for engineering projects such as road and utility construction (engineers mapping pipelines). They require very precise information measured to the millimeter
• A Demographic analyses of electoral trends could probably make do with slightly lower levels of attribute accuracy.

Question 13

Errors with accuracy and precision.

Answer 13

• Data quality: refers to the relative accuracy and precision of a particular GIS database. These facts are often documented in data quality reports.
• Error involves both the imprecision of data and its inaccuracies.

Question 14

What are common error types?

Answer 14

1. Positional accuracy and precision: horizontal & vertical location
2. Attribute accuracy (correct) and precision (enough detail info)
3. Conceptual accuracy and precision: the user determines classes and categories
4. Logical accuracy and precision: data must be used correctly…GIS systems are typically unable to warn the user if data are being used incorrectly.

Question 15

Examples of other errors.

Answer 15

1. Age of data ..too old?
2. Areal Cover ..Incomplete, like RS in cloudy areas
3. Map Scale … match the appropriate scale to the level of detail required in the project
4. Density of Observations
   too small test sample
5. Relevance …wrong data/surrogate data
6. Format - Conversion of scale, projection, changing from raster to vector format, and resolution size of pixels.
   - Cost often require data reformation to the “lowest common denominator” for
   transmission and use by multiple GIS
7. Accessibility - Restricted, classified data
8. Cost - SPOT VS Landsat

Question 16

What is positional accuracy?

Answer 16

• Variance of map features vs true position
• Depends on type of data, i.e. Discrete vs Continuous (often more difficult to map)

Question 17

Accuracy of content

Answer 17

• Complete? Correctly labelled?
• Attribute errors because of human error or faulty instrument calibration used to measure specific features such as altitude, soil or pH.

Question 18

Sources of variation in data

Answer 18

• Natural variation in data being collected ….. salinity in bays and estuaries dependent upon freshwater influx and evaporation

Question 19

Numerical errors

Answer 19

computer proccessing errors.

Question 20

Errors in overlay analysis

Answer 20

overlaying multiple layer of maps can result in problems such as Slivers, Overshoot, and Dangles.

Question 21

Classification and generalisation Problems

Answer 21

• For the human mind to comprehend vast amounts of data it must be classified, and in some cases generalized, to be understandable.

Question 22

Digitalising and geocoding Errors

Answer 22

Operator errors, and damaged source maps.

Question 23

What is error propagation?

Answer 23

occurs when one error leads to another i.e. error build up.

check metadata for error propagation.

Question 24

What is cascading?

Answer 24

means that erroneous, imprecise, and inaccurate information will skew a GIS solution.