Real-World Applications of COLUMBUS

Mixing GPS and Terrestrial Measurements in a Network

• 3D Network Adjustments
  
• 2D Network Adjustments
  
• 1D Vertical Network Adjustments

You can freely mix both GPS and terrestrial observations in a 3D network adjustment. A total of 19 observation types can be used:

• GPS dX, dY, dZ
• Azimuth
• Direction
• Horizontal angle right
• Bearing (true bearings or average bearings)
• Zenith angle
• Horizontal distance
• Chord (slope) distance
• Geodesic distance
• Geo Chord distance
• Local horizon dN, dE, dUp
• Height difference
• Latitude coordinate (geodetic adjustment only)
• Longitude coordinate (geodetic adjustment only)
• Height coordinate (geodetic adjustment only)

Terrestrial observations are normally leveled in the direction of gravity (Astronomic observations), while the geodetic model is "leveled" in the direction of the ellipsoidal normal (Geodetic observations). For high accuracy 3D geodetic surveys, you can correct the applicable terrestrial observations to the geodetic model by providing deflections of the vertical at each geodetic station. (When the deflection N-S and E-W are set to 0.0, no correction is applied.)

In the United States, these values can be determined using the COLUMBUS Deflection Modeling tool (from the Tools menu, select Deflection Modeling). Under most conditions, the zenith angle observation type usually gets the largest correction when deflection of vertical corrections are applied.

For many surveys, you may discover that applying this correction will not change your adjusted results by much. When possible, we encourage you to attempt your adjustments both ways: with and without deflection of the vertical corrections applied.

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2D Network Adjustments

For a 2D network adjustment, all the above observations are available except:

• GPS dX, dY, dZ
• Height difference
• Local horizon dUp
• Height coordinate

2D networks are adjusted at an average project height.

Zenith angles are only used when accompanied by a chord (slope) distance. The chord distance is then reduced to a horizontal distance (at the average project height). Note: The chord distance must have an applicable zenith angle unless the chord distance is a horizontal distance.

Due to the reductions described above and the fact that this adjustment type is carried out at an average project height, no deflection of the vertical corrections are applied. However, the accuracy of the model in 2D is quite strong provided there is not great variability in the terrain for the project area (in other words, large variations in elevation).

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1D Vertical Network Adjustments

For 1D vertical network adjustments the available observation types are:

• Height difference
• Local horizon dUp
• Height coordinate

Most vertical networks are made up of the Height Difference observation type. The Local Horizon Delta Up type is treated the same as the height difference observation type.

When 1D vertical networks are loaded, they are always displayed as a spiral. The spiral effect allows the user to see the relationships between stations (in other words, which stations are connected to one another). Click here for more information on networks displayed as a spiral.