GIS in Water Resources

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GIS in Water Resources

• Review for Midterm Exam

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Latitude and Longitude in North America
Austin (30N, 98W) Logan (42N, 112W)
60 N
30 N
60 W
120 W
90 W
0 N
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Length on Meridians and Parallels
(Lat, Long) (f, l)
Length on a Meridian AB Re Df (same for all
latitudes)
R
Dl
D
R
30 N
C
B
Re
Df
0 N
Re
Length on a Parallel CD R Dl Re Dl Cos
f (varies with latitude)
A
4

• Example What is the length of a 1º increment
  along
• on a meridian and on a parallel at 30N, 90W?
• Radius of the earth 6370 km.
• Solution
• A 1º angle has first to be converted to radians
• p radians 180 º, so 1º p/180 3.1416/180
  0.0175 radians
• For the meridian, DL Re Df 6370 0.0175
  111 km
• For the parallel, DL Re Dl Cos f
• 6370 0.0175
  Cos 30
• 96.5 km
• Parallels converge as poles are approached

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Horizontal Earth Datums

• An earth datum is defined by an ellipse and an
  axis of rotation
• NAD27 (North American Datum of 1927) uses the
  Clarke (1866) ellipsoid on a non geocentric axis
  of rotation
• NAD83 (NAD,1983) uses the GRS80 ellipsoid on a
  geocentric axis of rotation
• WGS84 (World Geodetic System of 1984) uses GRS80,
  almost the same as NAD83

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Vertical Earth Datums

• A vertical datum defines elevation, z
• NGVD29 (National Geodetic Vertical Datum of 1929)
• NAVD88 (North American Vertical Datum of 1988)
• takes into account a map of gravity anomalies
  between the ellipsoid and the geoid

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Coordinate System
A planar coordinate system is defined by a
pair of orthogonal (x,y) axes drawn through an
origin
Y
X
Origin
(xo,yo)
(fo,lo)
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Universal Transverse Mercator

• Uses the Transverse Mercator projection
• Each zone has a Central Meridian (lo), zones are
  6 wide, and go from pole to pole
• 60 zones cover the earth from East to West
• Reference Latitude (fo), is the equator
• (Xshift, Yshift) (xo,yo) (500000, 0) in the
  Northern Hemisphere, units are meters

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UTM Zone 14
-99
-102
-96
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Origin
Equator
-120
-90
-60
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ArcInfo 8 Reference Frames

• Defined for a feature dataset in ArcCatalog
• Coordinate System
• Projected
• Geographic
• X/Y Domain
• Z Domain
• M Domain

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X/Y Domain
(Max X, Max Y)
Long integer max value of 231 2,147,483,645
(Min X, Min Y)
Maximum resolution of a point Map Units /
Precision e.g. map units meters, precision
1000, then maximum resolution 1 meter/1000 1
mm on the ground
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Network Definition

• A network is a set of edges and junctions that
  are topologically connected to each other.

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Edges and Junctions

• Simple feature classes points and lines
• Network feature classes junctions and edges
• Edges can be
• Simple one attribute record for a single edge
• Complex one attribute record for several edges
  in a linear sequence
• A single edge cannot be branched

No!!
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Polylines and Edges
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Junctions

• Junctions exist at all points where edges join
• If necessary they are added during network
  building (generic junctions)
• Junctions can be placed on the interior of an
  edge e.g. stream gage
• Any number of point feature classes can be built
  into junctions on a single network

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Connectivity Table
p. 132 of Modeling our World
J125
Junction
Adjacent Junction and Edge
E2
J124
E3
E1
J123
J126
This is the Logical Network
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Flow to a sink
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Eight Direction Pour Point Model
Water flows in the direction of steepest descent
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Flow Direction Grid
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Cell to Cell Grid Network Through the Landscape
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Contributing Area Grid
Drainage area threshold gt 5 Cells
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Delineation of Streams and Watersheds on a DEM
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Stream Segments
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Stream Segments in a Cell Network
5
5
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Watershed Outlet
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Watershed Draining to This Outlet
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Watershed and Drainage Paths Delineated from 30m
DEM
Automated method is more consistent than hand
delineation
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1000 Cell Threshold Exceeded at Stream Junction
989
510
1504 (gt1000)
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Subwatersheds for Stream Segments
Same Cell Value
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Vectorized Streams Linked Using Grid Code to Cell
Equivalents
Vector Streams
Grid Streams
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Delineated Subwatersheds and Stream Networks
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A Mesh of Triangles
Triangle is the only polygon that is always
planar in 3-D
Lines
Surfaces
Points
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Tin Triangles in 3-D
(x3, y3, z3)
(x1, y1, z1)
(x2, y2, z2)
z
y
Projection in (x,y) plane
x
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Delauney Triangulation
Maximize the minimum interior angle of
triangles No point lies within the circumcircle
of a triangle
Yes
No
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Flow On a Triangle
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Flow On a TIN