Ch 3 Geospatial

Question 1

What is reprojection? Why is reprojection necessary when working with multiple geospatial data layers that are in different map projections?

Answer 1

To change a dataset from one map projection/measurement system to another

It is necessary so that measurements can match up

Question 2

Why doesn’t a scanned historical map match up with georeferenced data layers (e.g., a georeferenced satellite image)? What technique do you use to solve this issue?

Answer 2

Image doesn’t contain any spatial reference

I would assign real-world set of coordinates to my data

Question 3

Explain three major steps in georeferencing. Explain how each step is done

Answer 3

Step 1: Choose Source
Has to be published

Step 2: Select ground control points
More of them the better

Step 3: Transformation
Make the map Align with source image

Question 4

What are the standards of choosing a georeferencing source? What is a commonly used source for georeferencing?

Answer 4

Has to be scientifically reliable

Google Earth Pro

Question 5

What are ground control points (GCPs)? What are the standards of selecting GCPs?

Answer 5

Ground Control Points are point locations where coordinates are known and tie unreferenced data to source data

Good GCP Points

Question 6

What are good/poor examples of GCPs?

Answer 6

Good GCP: corners of buildings, intersections, ancient landmark

Bad GCP: Cars, shoreline of beach, Center of building/field

Question 7

What are the four transformation methods?

Answer 7

Rotation: turned angle around a fixed point

Skew: Slant an image

Translation: Shift an Image

Scale: change size

Question 8

What is the indicator/quantity used to evaluate how well a georeferenced image matches with the source? How to interpret this indicator?

Answer 8

Root Mean Square Error, Must be lower than 3 close to zero as possible

Question 9

What is the minimum number of GCPs required for georeferencing?

Answer 9

Three