Snow Depth and Temperature

1
Snow Depth and Temperature
100 m
150 m
100 m
150 m
4
6
2
4
Initial Snow Depth (cm)
Temperature (º C)
2
New depth calculation using Raster Calculator

• snow100m - 0.5 temp150m

3
The Result

• Outputs are on 150 m grid.
• How were values obtained ?

38
52
41
39
4
Nearest Neighbor Resampling with Cellsize Maximum
of Inputs
40-0.54 38
55-0.56 52
38
52
42-0.52 41
41-0.54 39
41
39
5
Controlling the scale of the output
The scale triplet
a) Extent
b) Spacing
c) Support
From Blöschl, G., (1996), Scale and Scaling in
Hydrology, Habilitationsschrift, Weiner
Mitteilungen Wasser Abwasser Gewasser, Wien, 346
p.
6
Spatial Analyst Options
Extent
Spacing Support
7
Raster Calculator Evaluation of temp150
4
6
6
6
4
4
4
2
4
2
2
4
4
Nearest neighbor to the E and S has been
resampled to obtain a 100 m temperature grid.
8
Raster calculation with options set to 100 m grid

• snow100m - 0.5 temp150m

• Outputs are on 100 m grid as desired.
• How were these values obtained ?

9
100 m cell size raster calculation
40-0.54 38
50-0.56 47
55-0.56 52
42-0.52 41
38
52
47
47-0.54 45
43-0.54 41
41
45
41
42-0.52 41
6
44-0.54 42
39
41
42
41-0.54 39
2
4
4
Nearest neighbor values resampled to 100 m grid
used in raster calculation
4
10
What did we learn?

• Spatial analyst automatically uses nearest
  neighbor resampling
• The scale (extent and cell size) can be set under
  options
• What if we want to use some other form of
  interpolation?

11
Find Temperature for each 100m cell
6
4
100m
150m
4
2
12
Zonal Statistics
Value shown is the mean in each 100m polygon of
the 10m grid values it overlays
13
Convert Features to Raster
100 m grid with mean Temperature value in each
cell
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Snow Computation
Dnew Dold 0.5 T
T
Dold
15
Calculating new snow depths
Use raster calculator to get grid, multiply grid
by 10 and convert to integer grid, then convert
to polygon, then divide value attribute by 10 to
get display
16
Summary of Snow Computation
Dnew Dold 0.5 T 38 40 0.5 4
Dold
Dnew
T
17
Some things about Grids.

• Value attribute table
• Conversion from rasters to polygons
• Editing grids??

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Value Attribute Table

• A record for each unique value that appears in
  the grid
• Value is the value in the grid
• Count is the number of cells containing this
  value

temp
19
Convert Cells to Polygons
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Temperature Polygons
4 cells make 4 polygons because none of the
cells with common values have common sides.
Temppoly.shp
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Snowpoly
9 cells converts to 8 polygons because the two
cells with 42 have a common side
22
Can Edit Only Features not Rasters
23
Homework 3 Models
Does snowmelt calculations on 100m grid
Creates 100m polygons and Temperatures in them
using a 10m grid

• Can t mix up models whose fundamental grid cell
  size is different
• Run Temperature first
• Then run Snowmelt

24
Temperature Model
Input Temp.txt Outputs Temp150m (Grid),
Poly100m.shp, Tempstat.dbf
25
Temperature grids
ncols 2 Nrows 2 xllcorner 0 yllcorner 0 cellsize
150 NODATA_value -9999 4 6 2 4
26
Create a random raster
With raster environment same as snow
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Creating Unique 100m polygons
Create a random raster with 100m zones whose cell
values are randomly generated and multiplied by
1000 to get unique integers
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Zonal Statistics Table
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Snowmelt Model
Inputs TempStat.dbf, Poly100m.shp,
Snow.txt Outputs Snow100m, Temp100m, NewSnow
(Grids)
Default cell size 100m
30
100m Temp Grid
Use ID as Join Field
Use Mean as value to be converted to raster
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Snowmelt
Dnew Dold 0.5 T 38 40 0.5 4
ncols 3 Nrows 3 xllcorner 0 yllcorner 0 cellsize
100 NODATA_value -9999 40 50 55 42 47 43 42 44 41
32
Rules for Working with Rasters

• A given model can have only one default cell size
• Raster cells can be converted to Feature polygons
  provided raster values are integer
• Zonal statistics can be used to summarize raster
  values within Feature polygons provided that the
  cell size is smaller than the polygon size.
• ArcToolbox tool Create Random Raster can be used
  to create rasters with unique values in each cell