Study Guide ArcGIS Imagery MOOC
The goal is to understand the concepts well enough to eliminate incorrect choices rather than memorize the quiz.
Study Guide — Quiz: Section 1
This quiz is testing four main areas:
- Hyperspectral vs. multispectral imagery
- Spectral signatures
- Dynamic Range Adjustment (DRA)
- Radar imagery and open geospatial services
The goal is to understand the concepts well enough to eliminate incorrect choices rather than memorize the quiz.
1. Hyperspectral Imagery (HSI)
Core concept
Remote sensing instruments measure electromagnetic radiation across different parts of the electromagnetic spectrum (EMS).
The important distinction is the number and width of spectral bands.
| Imagery type | Spectral bands | Typical characteristic |
|---|---|---|
| Panchromatic | 1 broad band | Grayscale image |
| Multispectral (MSI) | Several bands | Relatively broad bands |
| Hyperspectral (HSI) | Many bands | Narrow, closely spaced bands |
Think of it this way
Imagine the electromagnetic spectrum as a ruler.
Multispectral:
1
2
|----| |------| |----|
band band band
There are relatively few, broader sections.
Hyperspectral:
1
|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|
The spectrum is divided into many narrow sections.
Key distinction to remember
When you see:
- many
- narrow
- closely spaced
- continuous/near-continuous spectral coverage
→ think hyperspectral.
When you see:
- few/several
- broader
- discrete bands
→ think multispectral.
How to solve Question 1
Don’t focus on the exact wording of the choices initially.
Ask:
Is the question describing a sensor with a few broad spectral measurements or a very detailed spectral sampling?
Then match that description to MSI or HSI.
2. Spectral Signatures and Spectral Profiles
Question 2 tests whether you understand what a spectral signature represents.
What is a spectral signature?
Different materials interact with electromagnetic radiation differently.
For example:
- vegetation
- water
- soil
- concrete
- minerals
can have different patterns of reflection at different wavelengths.
A spectral signature is essentially the characteristic relationship between:
wavelength → reflectance
Typical spectral profile
A graph might look conceptually like:
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Reflectance
^
| /\____
| _____/ \
| ___/
|__/
+--------------------------> Wavelength
There are two important pieces of information:
X-axis
Usually represents:
wavelength
Examples:
- 450 nm
- 550 nm
- 650 nm
- 850 nm
Y-axis
Usually represents:
reflectance
This describes how much incoming radiation is reflected by the material.
Important distinction: wavelength vs. frequency
Wavelength and frequency are related:
\[c = \lambda f\]where:
- (c) = speed of light
- (\lambda) = wavelength
- (f) = frequency
So wavelength and frequency describe electromagnetic radiation, but when interpreting a spectral signature in remote sensing, you should pay attention to what quantity the profile normally uses on each axis.
How to solve Question 2
The question asks:
What two types of data are plotted?
Think of a graph:
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?
^
|
|
+------------------> ?
Ask yourself:
- What goes along the horizontal axis?
- What is being measured vertically?
Don’t select based simply on whether something is related to electromagnetic radiation. Identify the variables actually used in a spectral profile.
3. Dynamic Range Adjustment (DRA)
This is an important GIS/image-display concept.
What is dynamic range?
Raster imagery contains numerical pixel values.
For example, imagine an image with values:
1
2
3
10 12 15 18
20 25 30 35
40 50 60 70
Your display device has a limited range of brightness values that it can display.
The GIS software therefore needs to map the raster’s values onto the available display range.
What does DRA do?
Dynamic Range Adjustment dynamically determines the range used for displaying the imagery.
The important question is:
What area of the image is considered when determining the display range?
Compare these possibilities conceptually:
A. A manually selected polygon
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+-----------------------+
| |
| ________ |
| / \ |
| / selected\ |
| \ area / |
| \________/ |
| |
+-----------------------+
B. The current map/display extent
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+-----------------------+
| CURRENT MAP |
| EXTENT |
| |
| |
+-----------------------+
C. The entire raster
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+-----------------------+
| |
| ENTIRE |
| IMAGE |
| |
+-----------------------+
The key is to understand what DRA uses as its basis for calculating the displayed range.
How to solve Question 3
Look carefully at the wording:
“DRA is currently enabled”
and especially:
“affects the display of imagery on a map”
This tells you that the question is about display rendering, not changing the underlying raster data.
Ask:
- Is DRA permanently modifying the pixel values?
- Or is it determining how the pixels are rendered?
- What portion of the imagery is relevant to the current rendering?
Important distinction
Rendering ≠ changing the source data.
A display adjustment can change how an image looks without changing the original raster values.
4. Radar and Cloud Cover
Question 4 is testing your understanding of active vs. passive remote sensing and, specifically, why radar is useful when optical imagery is affected by clouds.
Optical imagery
Multispectral and hyperspectral sensors generally depend on electromagnetic radiation in portions of the spectrum where atmospheric conditions can strongly affect observations.
Clouds can therefore cause a major problem.
Conceptually:
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CLOUDS
☁ ☁ ☁ ☁ ☁
----------------
forest
🌳 🌳 🌳 🌳 🌳
The sensor may not be able to obtain a useful observation of the surface.
Synthetic Aperture Radar (SAR)
SAR is a form of active remote sensing.
Instead of simply measuring naturally reflected sunlight, radar systems transmit electromagnetic energy and measure the returned signal.
Conceptually:
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SAR
↓
↓↓↓
☁ ☁ ☁ ☁ ☁
↓ ↓ ↓ ↓ ↓
🌳 🌳 🌳 🌳 🌳
↑ ↑ ↑ ↑ ↑
returned
signal
Radar operates at microwave wavelengths, which have characteristics that make SAR particularly useful for observations through cloud cover and in darkness.
Active vs. passive
This distinction is worth memorizing:
| Type | Basic idea |
|---|---|
| Passive optical sensor | Measures naturally available radiation |
| Active radar | Sends out its own signal and measures the return |
SAR is especially useful for
- cloud-covered regions
- nighttime observations
- frequent monitoring
- vegetation/forest analysis
- surface structure
- deformation
- floods
- some soil/moisture applications
How to solve Question 4
Focus on the constraint:
continuous environmental monitoring + frequent cloud cover
Then ask:
Which sensor is least dependent on clear skies and sunlight?
This is a classic sensor-selection question.
Do not choose based on which technology sounds more advanced. Match the sensor’s physical properties to the environmental problem.
5. Open Geospatial Services
Question 5 changes topic slightly. It tests your knowledge of web-based geospatial data/services.
You need to distinguish between:
- a GIS platform
- a GIS content store
- a web mapping service
- an actual geospatial web service standard
OGC and open standards
The Open Geospatial Consortium (OGC) develops standards that allow geospatial systems to communicate.
Examples include services for:
- maps
- geographic features
- coverages
The important idea is:
An open geospatial service can allow different software systems to access geographic information using standardized interfaces.
This is particularly important for interoperability.
WMS / WMTS / WCS
You should know the conceptual differences.
WMS — Web Map Service
Provides maps/images generated from geographic data.
Think:
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GIS
↓
WMS
↓
Map image
The client is essentially asking:
“Give me a map of this area.”
WMTS — Web Map Tile Service
Provides map content as tiles.
Think of a web basemap divided into many small pieces:
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+---+---+---+
| | | |
+---+---+---+
| | | |
+---+---+---+
| | | |
+---+---+---+
The client requests the tiles it needs.
WCS — Web Coverage Service
This one is important because it is different from simply requesting a rendered map.
A coverage represents spatially distributed data, such as raster-type information.
Think:
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WMS/WMTS
→ "Show me the map"
WCS
→ "Give me the actual coverage data"
The exact capabilities depend on the service implementation and standard version, but this distinction is useful for quiz questions.
6. Don’t Confuse a Service With a Platform
Question 5 includes names that belong to very different categories.
Build this mental classification:
| Category | What to ask |
|---|---|
| GIS platform | Is this a software/platform ecosystem? |
| Online GIS | Is this a hosted/cloud GIS environment? |
| Content store | Is this a repository/catalog of GIS content? |
| Web service | Is this a standardized mechanism for accessing geospatial information? |
The question specifically asks about open-source/open geospatial layers, so don’t simply choose something because it is available through the internet.
Key reasoning principle
“Online” does not automatically mean “open standard.”
And:
“Available in ArcGIS” does not automatically mean “ArcGIS technology.”
7. What You Should Memorize
For this quiz, create this small mental cheat sheet:
Remote sensing
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MSI
↓
Several broader bands
HSI
↓
Many narrow bands
Spectral profile
1
2
X → wavelength
Y → reflectance
DRA
1
2
3
4
5
DRA
↓
display/rendering
↓
determines how pixel values are mapped
Focus on what spatial extent is used for the display calculation.
SAR
1
2
3
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SAR
↓
active sensor
↓
microwave
↓
works day/night
↓
less affected by cloud cover
Web geospatial services
1
2
3
4
5
WMTS → map tiles
WMS → map imagery
WCS → coverage/raster-type data
And distinguish these services from GIS platforms and content repositories.
8. Recommended Way to Attack Each Question
Before looking at the choices, identify the keyword that determines the answer.
| Question | Key concept | Ask yourself |
|---|---|---|
| 1 | HSI | How many/how narrow are the spectral bands? |
| 2 | Spectral signature | What are the two graph axes? |
| 3 | DRA | How does the current display determine its rendering range? |
| 4 | SAR | Which sensor can monitor despite cloud cover? |
| 5 | Open geospatial services | Which options are actual standardized web-service technologies? |
Then use elimination.
For example, if a question asks about HSI, don’t immediately search for the answer. Eliminate options that describe:
- too few bands
- excessively broad bands
- unrelated portions of the electromagnetic spectrum
This makes the quiz much easier without relying on memorization.
Final study checklist
Before taking the quiz, make sure you can explain these without looking at notes:
- Difference between multispectral and hyperspectral
- Meaning of spectral signature
- Difference between wavelength and frequency
- Why spectral profiles use wavelength and reflectance
- What dynamic range means in raster display
- Difference between rendering and modifying raster data
- Difference between active and passive remote sensing
- Why SAR is useful with cloud cover
- Difference between WMS, WMTS, and WCS
- Difference between a web service, GIS platform, and content store
If you can explain those ten points in your own words, you should be able to solve the five questions without memorizing the answer choices.