Remote Sensing and GIS (Year 2) - Part 1 GIS Flashcards Preview

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Flashcards in Remote Sensing and GIS (Year 2) - Part 1 GIS Deck (90)
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Direct observation-> E.g. ground survey
What is type of data collection is this ?



- Digitising data collected by someone else
- E.g. digitising a paper map
- Digitisation of existing data
o Manual digitising
o Digitising tablets
o Photogrammetry
What is type of data collection is this?



What type of data is this (discrete or continuous)?
- Data that have distinct spatial extent
- E.g. roads, buildings, fields



What type of data is this (discrete or continuous)?
- Data that occur everywhere (data at every point on the map)
- E.g. atmospheric pressure, height, air temperature



What are the two models used by computers to represent the world (types of data)?

Raster and Vector


What coordinate system is used by vector data?

Uses Cartesian coordinates (x, y) to represent spatial entities


What are the 5 types of data within the vector model?

• Point is simplest entity -> single (x, y) pair
• Lines -> built by connecting multiple points (each is an x,y point joined together)
• Polygons – closed lines
• Networks – connected lines that don’t form a polygon e.g: Road network or River network
• Surface – mesh of coordinates -> e.g. elevations


What are the attributes of vector data?

• Spatial entities are linked to their attributes
• The attribute table contains this attribute information
• This is what enables us to do analysis


What are the limitations of vector data sets?

• Polygons along shared boundaries (inefficient)
• No information about links in networks
• No information about objects within other objects


What technique can be used to overcome the issues of polygons not being linked in vector data?

Topology which ensure there is connection between entities (topological editing).


How does the raster model display data? (attributes)

• Space (entities, surfaces) represented by tessellations
• array of grid cells (usually square)
• Attributes (i.e. values) assigned to these cells define entities and surfaces
• Each cell has a single value representing the attribute


How do you make the raster display more accurate?

Increase the resolution (decrease the size of each grid cell)


What is the issue with increasing the resolution (raster data)?

Doubling the resolution increases storage requirement by four times -> increasing resolution = more storage required


What is 'run length' an example of?

A computer strategy used to decrease the storage requirement for raster data


What techniques are used with raster data when a cell contains more than one attribute? Explain them

- Greatest area allocation (>1 lines/polygons in one cell)
- Centre point allocation (>1 points in one cell)


What is vector data good for? (type of data)

Features that have discrete boundaries (e.g. a building)


What is raster data good for? (type of data)

• Continuous geographic data (e.g. temperature)
• Spatial analysis and modelling
• Data collected on a regular grid (e.g. satellite data)


What are digital terrain models (DTM)?

Height of the land surface in relation to a datum


What are digital terrain models (DTM) also known as?

Sometimes referred to as a digital elevation model (DEM)


What is an elevation model that includes man-made features known as?

(Digital Surface Model)


What are DTM's based on?

Based on height samples


Why do we simplify the landscape for making DTMs?

We simplify our landscape as we can’t have a value for every point


What does the number of height values in the model depend on?

• The purpose of the model
• The finite data storage available


What are the two ways in which DTM can be collected?

Primary and Secondary


What are the primary data techniques to collect data to form DTMs?

• Ground survey (e.g. GPS, total station) - Time consuming (expensive) + Can be very accurate (a few mm precision) – surveyors use this
• Photogrammetry - Broad set of techniques regarding the taking of photographs -> e.g. taken from different angles and heights + Can have very high spatial/vertical resolution
• Satellite (radar) altimetry - Tends to have low spatial resolution
• SAR (Synthetic Aperture Radars) Interferometry (InSAR) - Can be of very high resolution (mm)
• LiDAR (light detecting and ranging)– use of light beams and measuring the time it takes to return e.g. through the use of a plane


Globally what resolution of DEM is available for public use?

30 metre


At which regions are there higher resolution of DEM available?

• 2 m for the Arctic (high resolution – lots of data)
• UK wide – 5 m
• UK selected areas – LiDAR derived DEM sub meter resolution (e.g. 50 cm for selected parts of England)


How is secondary data used to form DTMs?

Based on other's data, e.g. digitising contours on maps


What are the issues of using secondary data to form DTMs?

• Time consuming
• Lack of data in relatively flat areas (wide contour spacing)
• Only as good as the map (accuracy/resolution)


How does the raster approach represent surfaces in GIS? (DTM)

Elevation represented by a regular grid of numbers