ENVSOCTY 3GI3 EXAM QUESTIONS & ANSWERS
Modelling flow of water across a surface, is based on: - Answer -downslope flow
derived from a DEM *looks at surface flow
Watershed - Answer -area of land that collects water (drains to this one point)
ArcGIS can model drainage systems with: - Answer -dendritic morphology with
assumption that gravity is main driver for flow
Hydrology - DEM resolution: Continental or Provincial Scale - Answer -100m or greater
Hydrology - DEM resolution: Regional Scale - Answer -30m
Hydrology - DEM resolution: Smaller sub-basins - Answer -1-10m
________________ and _______________ increased with finer resolution. - Answer -
feature complexities, processing time
* thus, have to consider and trade-off
Enriching the information content of a DEM (3): - Answer -- Identify and correct
possible errors in the DEM (hydrologic preconditioning)
- Deriving runoff characters (calculating flow based on derivative surfaces -> stream
networks and characteristics)
- Delineation of watersheds
Hydrologic Preconditioning Workflow - Reduce possibilities of error - Answer -- Input
DEM
- Flow direction (what direction water flow out of cell)
- Sink tool (may be natural but typically error)
— If Yes, use the fill tool
- Result: Hydrologically conditioned DEM
Flow direction is derived from DEM by performing a: - Answer -Focal Function
Flow from cell is based upon: - Answer -Aspect (direction of maximum slope)
- For focal cell, this direction is determined by evaluating equation 8 times (b/c 8 cells
surround), and selecting the maximum drop
Focal Cell Flow Direction Equation - Answer -Drop = (elevation of focal cell - elevation
of neighbouring cell)/(horizontal distance)
- The greater the value the steeper the drop
Flow Direction output - Answer -Grid of flow direction from each cell to its steepest
downslope neighbour
,3 ArcGIS Flow Direction Algorithms - Answer -- D8 — cell centre to cell centre
- D INF — any angle
- MFD - multi-flow direction
D8 - Answer -book keeping scheme
- solely determines direction of flow
D INF - Answer -stores value as a single angle in degrees going counterclockwise from
0 (due east)
MFD - Answer -partitions from cell to all downslope neighbours, stored as a cloud
raster formed (CRF)
What are a concern in many analyses involving raster datasets? - Answer -Edge
Effects
For Flow Direction, what are the 2 possible situations at the edges? - Answer --
Normal: cells that should flow inward are coded as such (if maximum drop is <0, then
cell flows out of the DEM)
- Force: all cells at the edge flow out of the DEM
2 DEM Errors - Answer -- Sinks
- Peaks
Sinks - Answer -- cell(s) surrounded by cells with higher elevation
- consequence: no direction of steepest descent
- in general, more sinks in worse DEMs
Peaks - Answer -- cell surrounded by cells with lower elevation
To fix sinks: - Answer -- Identified using the SINK command
- Filled used the FILL command (one cell sinks are assigned elevation of lowest
neighbour, thus allowing direction to be computed)
To fix peaks: - Answer -- removed by assigning elevation of next highest neighbour
Complete workflow to identify and remove sinks: - Answer -- Determine flow direction
using FLOWDIRECTION function
- Locate sinks in flow direction grid using the SINK function
- Correct identified sinks using the FILL command
- Recompute FLOWDIRECTION with the now depressionless DEM
When are sinks potentially now errors to be filled? - Answer -- Lakes
- Depressions
- Karst landscapes
, - Glacial landscapes
If the same change in elevation in multiple directions is observed and the focal cell is a
sink, then direction is: - Answer -undefined being the sum of the tied directions using
the D8 algorithm
FILL uses the equivalent of the following tools in its search algorithm (5): - Answer --
Focal Flow
- Flow Direction
- Sink
- Watershed
- Zonal Fill
FILL tools iterates until: - Answer -all sinks are filled within a user specified Z-limit (this
yields a 'selective' fill)
Flow Length Tool - Answer -Calculates flow distance for every cell to the pour point
Flow Accumulation - Answer -Flow direction grid used to produce flow accumulation,
the area draining into each downslope grid cell
- Where water is most likely to accumulate
Flow Acc >= 5 - Answer -Identifies potential stream cells in a grid
Use _______ or _______ to create new stream network - Answer -con, setnull
STREAMLINK - Answer -assigns unique number to each stream network
- can be used to create watersheds
Stream Ordering - Answer -Used to define size of streams base on a hierarchy of
tributaries
2 Options for stream ordering - Answer -- Strahler
- Shreve
Strahler Stream Ordering - Answer -- Stream order only increases when streams of the
same order intersect
- Ex. Combination of 2 and 2 makes 3
Shreve Stream Ordering - Answer -- All links with no tributaries are assigned an order
of 1
- Orders are additive downslope
- When 2 links intersect, their orders are added and assigned to the downslope link
Watershed Delineation - Answer -- Watersheds can be delineated for an entire grid or
for selected points (pour points)
Modelling flow of water across a surface, is based on: - Answer -downslope flow
derived from a DEM *looks at surface flow
Watershed - Answer -area of land that collects water (drains to this one point)
ArcGIS can model drainage systems with: - Answer -dendritic morphology with
assumption that gravity is main driver for flow
Hydrology - DEM resolution: Continental or Provincial Scale - Answer -100m or greater
Hydrology - DEM resolution: Regional Scale - Answer -30m
Hydrology - DEM resolution: Smaller sub-basins - Answer -1-10m
________________ and _______________ increased with finer resolution. - Answer -
feature complexities, processing time
* thus, have to consider and trade-off
Enriching the information content of a DEM (3): - Answer -- Identify and correct
possible errors in the DEM (hydrologic preconditioning)
- Deriving runoff characters (calculating flow based on derivative surfaces -> stream
networks and characteristics)
- Delineation of watersheds
Hydrologic Preconditioning Workflow - Reduce possibilities of error - Answer -- Input
DEM
- Flow direction (what direction water flow out of cell)
- Sink tool (may be natural but typically error)
— If Yes, use the fill tool
- Result: Hydrologically conditioned DEM
Flow direction is derived from DEM by performing a: - Answer -Focal Function
Flow from cell is based upon: - Answer -Aspect (direction of maximum slope)
- For focal cell, this direction is determined by evaluating equation 8 times (b/c 8 cells
surround), and selecting the maximum drop
Focal Cell Flow Direction Equation - Answer -Drop = (elevation of focal cell - elevation
of neighbouring cell)/(horizontal distance)
- The greater the value the steeper the drop
Flow Direction output - Answer -Grid of flow direction from each cell to its steepest
downslope neighbour
,3 ArcGIS Flow Direction Algorithms - Answer -- D8 — cell centre to cell centre
- D INF — any angle
- MFD - multi-flow direction
D8 - Answer -book keeping scheme
- solely determines direction of flow
D INF - Answer -stores value as a single angle in degrees going counterclockwise from
0 (due east)
MFD - Answer -partitions from cell to all downslope neighbours, stored as a cloud
raster formed (CRF)
What are a concern in many analyses involving raster datasets? - Answer -Edge
Effects
For Flow Direction, what are the 2 possible situations at the edges? - Answer --
Normal: cells that should flow inward are coded as such (if maximum drop is <0, then
cell flows out of the DEM)
- Force: all cells at the edge flow out of the DEM
2 DEM Errors - Answer -- Sinks
- Peaks
Sinks - Answer -- cell(s) surrounded by cells with higher elevation
- consequence: no direction of steepest descent
- in general, more sinks in worse DEMs
Peaks - Answer -- cell surrounded by cells with lower elevation
To fix sinks: - Answer -- Identified using the SINK command
- Filled used the FILL command (one cell sinks are assigned elevation of lowest
neighbour, thus allowing direction to be computed)
To fix peaks: - Answer -- removed by assigning elevation of next highest neighbour
Complete workflow to identify and remove sinks: - Answer -- Determine flow direction
using FLOWDIRECTION function
- Locate sinks in flow direction grid using the SINK function
- Correct identified sinks using the FILL command
- Recompute FLOWDIRECTION with the now depressionless DEM
When are sinks potentially now errors to be filled? - Answer -- Lakes
- Depressions
- Karst landscapes
, - Glacial landscapes
If the same change in elevation in multiple directions is observed and the focal cell is a
sink, then direction is: - Answer -undefined being the sum of the tied directions using
the D8 algorithm
FILL uses the equivalent of the following tools in its search algorithm (5): - Answer --
Focal Flow
- Flow Direction
- Sink
- Watershed
- Zonal Fill
FILL tools iterates until: - Answer -all sinks are filled within a user specified Z-limit (this
yields a 'selective' fill)
Flow Length Tool - Answer -Calculates flow distance for every cell to the pour point
Flow Accumulation - Answer -Flow direction grid used to produce flow accumulation,
the area draining into each downslope grid cell
- Where water is most likely to accumulate
Flow Acc >= 5 - Answer -Identifies potential stream cells in a grid
Use _______ or _______ to create new stream network - Answer -con, setnull
STREAMLINK - Answer -assigns unique number to each stream network
- can be used to create watersheds
Stream Ordering - Answer -Used to define size of streams base on a hierarchy of
tributaries
2 Options for stream ordering - Answer -- Strahler
- Shreve
Strahler Stream Ordering - Answer -- Stream order only increases when streams of the
same order intersect
- Ex. Combination of 2 and 2 makes 3
Shreve Stream Ordering - Answer -- All links with no tributaries are assigned an order
of 1
- Orders are additive downslope
- When 2 links intersect, their orders are added and assigned to the downslope link
Watershed Delineation - Answer -- Watersheds can be delineated for an entire grid or
for selected points (pour points)
