2017 GIS in the Rockies presentation

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USING ARCGIS AND A DIGITAL ELEVATION MODEL TO
CREATE A LOCAL COORDINATE SYSTEM
13 January 2017
AKA Low Distortion Projection
Teresa L Smithson, PLS, GISP, MSEng

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October 11, 2017
Jacobs Engineering Group Inc. All rights reserved.
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3. Agenda
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1. Effect of elevation on ground measurements
2. Acquiring digital elevation models (DEM)
3. Steps to creating multiple zones based on
elevation using ArcGIS

4. Effect of Elevation on Ground Measurements
• If the ellipsoid distance between two points is
1,000 meters (3,280.83 feet), the ground distance
at a height of:
– 1,000 meters is 1,000.16 meters;
– 1,500 meters is 1,000.24 meters;
– 2,000 meters is 1,000.31 meters;
– 2,500 meters is 1,000.39 meters.
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5. Systemic error
Establishment of a scale factor every:
– 100 meters (+/- 50m. from zonal median) 1:63,740
– 200 meters (+/- 100m. from zonal median) 1:31,880
– 400 meters (+/- 200m. from zonal median) 1:15,940
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6. Geoid Separation
• When defining a projection, the
ellipsoid height is used, not the
ground or orthometric elevation.
• For every 6 meters in height not
adjusted for (~20 feet), the
distortion is approximately 1 ppm.
– Ex. The geoid separation in Grand
Junction, Colorado is roughly -16
meters.
– Not adjusting to the ellipsoid will
increase distortion by almost 3 ppm.
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7. Download the DEM for the subject area
• USGS data at
https://viewer.nationalmap.gov/launch/
• Click on “Download GIS Data”
• Check “Elevation Products (3DEP)
• Check 1/3 arc-second DEM
• Zoom to subject area
• Click on Find Products
• Select the area/s desired
• Download to local drive
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8. DEM web page
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9. In ArcGIS, Start with the DEM
• Connect to the data folder
• Locate the DEM file and drag and
drop the DEM into ArcMap (pyramid
building is not required)
• Activate the Spatial Analyst extension
• “Reclassify” [Toolbox>Spatial Analyst
Tools>Reclass>Reclassify]
• Input raster = downloaded DEM
• Reclass Field = Value, and
• Output to the data folder, naming the
output raster …DEM_Reclass
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10. Reclassify DEM to create breaks
• Click on ‘Classify’,
• Set ‘Method’ = “Defined Interval”
• Set the size to the elevation range to
be used for each projection zone
• When done, the file will be added to
the display with each interval a
different color.
• Close the dialog box.
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11. Results of DEM Reclassify
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Note: as each step
adds a layer turn off
the previous layer.
This speeds up map
regeneration and
prevents selecting
the wrong
information.

12. Convert the elevations to integers
• Convert the elevation numbers to integers
• Use the INT tool [Spatial Analyst>Math>INT].
• Click on the INT tool, and run the program on the new DEM_Reclass
layer
• Output file is named …DEM_Int
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13. Convert the DEM to polygons
• Open the Raster to Polygon tool [Conversion Tools>From
Raster>Raster to Polygon],
• Input file = DEM_Int,
• Select VALUE for the field,
• Output file = DEM_polygons
• Check the “simplify polygons” box
• Click OK and run the tool
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14. Results of Raster to Polygon
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15. Add field for “Area”
A new field needs to be added to the polygon layer to clean
up the noise caused by the localized variations
• Right click on the polygon layer name and open the
attribute table.
• Click on options in the upper left corner and select Add a
field.
• Parameters are:
– Field Name = Area
– Type = Double
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16. Calculate areas
This is not a highly accurate measurement but it will only be
used for deciding how small or large we want the zones to
be. It is used to merge the tiny polygons into the larger ones.
• Right click on field Area and select calculate geometry.
• Select an appropriate area unit (I used square miles for my
measurement)
• Sort the area and select all of the records that are too
small to be considered separate LDP zones.
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17. Select small polygons for merging
• Here, all of the records
less than 0.80 square
miles were selected.
• Close the table but don’t
clear the selection.
• Look to be sure the
highlighted polygons are
the correct ones to
eliminate.
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18. Clean up the polygons
• Run the eliminate tool [Data
Management
Tools>Generalization>Eliminate]
to dissolve the selected polygons.
• Uncheck “Eliminate polygon by
border” in order to absorb the
small parts into the larger area
polygons.
• Output = …Poly_Eliminate.
• Click OK.
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19. Calculate the zone statistics
• Open the Zonal Stats as Table tool [Spatial Analyst>
Zonal>Zonal Stats as Table].
• Use the original DEM file for the raster input.
• Click OK.
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20. Add the stats as attributes
• Add a new attribute field to
Poly_Eliminate
• Parameters are:
– Field Name = ElevFactor
– Precision = 10, Scale = 9
• Join the stats table to Poly_Eliminate
• Option 1 is GRIDCODE.
• Option 2 is the stats table created.
• Option 3 is GRIDCODE.
• Select “Keep all records”. Click Ok.20

21. Add elevation factor to polygons
• Open the Poly_Eliminate layer attribute table and right click
on the new header “ElevFactor”.
• Use Field Calculator to enter the elevation factor
– using the median elevation factor from the joined stats table.
[median instead of the mean will assure 50% of the area is above
the reference elevation, and 50% will be below.]
– Enter in the field calculator “Elevation Factor (meters)= 6,371,000 /
(6,371,000 + Median Elev + geoid separation if known)”
• Close the table.
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22. ElevFactor in attribute table
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23. Define the point of origin
• Use the Median Center tool (Spatial Statistics Tool > Measuring
Geographic Distributions > Median Center) to calculate the
median center of each zone .
• Set the Case Field = zone ID (in this case GRIDCODE) to
consider separate polygons in the same zone as one zone.
• If working over a large area and want to treat all zones as having
separate points of origin, do not use this option.
• Output = polygon_median
• Click OK.
Note- if the area is very small then a
single point of origin can be used
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24. Add point of origin as attribute
• If not done previously, set the data frame coordinate
system to the geographic datum, NAD 1983 (2011)
• Add a field for the latitude and longitude, set
– type = double
– Field Name = Lat_NAD83, Precision = 13, Scale = 9
– Field Name = Long_NAD83, Precision = 13, Scale = 9
• Use Geometry Calculator to populate each field with the
NAD83 decimal degrees
• Close the table.
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25. Populate attribute field with geographic coordinates
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26. Format for exhibit
• Add labels to each zone in the Polygons_Median layer.
– Label expression is “Lat = “ & [Lat_NAD83] & vbCrLf & “ Long = “ &
[Long_NAD83]
• Add labels to the Eliminate polygon layer
– Set Label Field to “ElevFactor”.
• Set transparencies if a basemap will be used behind the
projection layers
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27. Final Result
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A false northing and easting needs to
be assigned to the point of origin that
provides a positive value for all points
within the project.
Ex: projection zone 2 (elev 1780 to
1900 meters) in yellow will be:
Datum – NAD 1983 (2011) (US Feet)
Projection – Transverse Mercator
Central Meridian (Longitude)
-105.190755307
Latitude of Origin
39.707976053
Scale Factor – 0.999713157
Rotation – 0
False northing* – 200,000
False Easting* – 100,000
(*large enough that the coordinates at
the SW corner are positive)

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