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Measuring Mid-Span Clearance with the ikeGPS Page 1 of 8

Measuring Mid-Span Clearance with the ikeGPS

Abstract

The ikeGPS provides a quick and straightforward method to accurately measure power line mid-span

clearance at road crossings without having to stop traffic or place the workers in the path of oncoming

vehicles. The Span Height function in the ikeGPS ikeCapture software utilizes a three step procedure for

capturing mid-span clearance.

Field tests determined that the ikeGPS locates the base point directly under the conductor within six

inches. Single readings of the mid-span clearance are accurate within 3% or less of the clearance height,

while averaging multiple mid-span clearance readings results in an accuracy of .5% of the clearance

height.

Traditional Methods

The most common method for measuring mid-span clearance in the telecom and power utility

industries has been an extendable fiberglass measuring pole (hot stick) that is extended and raised to

touch the conductor at the designed mid-span point. The height is read off the ruler on the measuring

pole and written down. This method has the following limitations and challenges:

The worker must stand directly under the mid-span measuring point, often requiring the worker

to stand in the path of oncoming traffic. In order to safely collect the mid-span clearance

measurement, additional manpower and time is required for traffic control.

Measuring poles wear out and can impact the accuracy of the height measurement.

No auditable verification of the work performed is available.

Limited to the height of the measuring pole, resulting in locations where the mid-span clearance

cannot be measured.

There are many locations where the extendable measuring pole cannot be used. For example, it can be

difficult if not impossible to block traffic on very busy highways. For these locations, either mobile or

airborne LiDAR has been used. While LiDAR measurements are accurate, they generate large amounts

of data, require significant post processing and data analysis and are considerably more expensive than

other methods, especially when data must be collected over a large number of geographically dispersed

locations.

ikeGPS Mid-span Clearance Measurements

Using the ikeGPS mid-span clearance measurements can easily be collected from the side of the road

without obstructing traffic or placing the field worker in an unsafe environment. The ikeGPS records the

location (latitude, longitude and altitude) of the mid-span measurement, the span clearance height and

generates a geo-referenced annotated image showing the clearance height and location of the


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measurement. There is no limitation on the mid-span clearance height that can be measured. The

ikeGPS can be used to measure clearance heights of 20 feet (7 meters) or clearance heights of 150 feet

(50 meters).

The ikeGPS ikeCapture Span Height mode is used to collect the mid-span clearance. The ikeGPS is

typically mounted on a non-magnetic tripod to provide additional stability during the measurementprocess. The Span Height measurement process has the following steps:

1. The ikeGPS is mounted on a tripod and is placed on the side of the road at a distance

approximately twice the span height from the conductor.

2. ikeCaptures Span Height mode is selected.

3. The conductors mid-span point is targeted in the ikeGPS crosshairs as shown in figure 1.

After waiting a couple of seconds to make sure the ikeGPS compass has stabilized and the

laser is locked on the conductor, the green arrow us tapped and the measurements process

proceeds to step 2.

Figure 1: Targeting the Wire


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4. In step 2, the ikeCapture assists the operator with finding the base point directly under the

mid-span target. ikeCapture does this by providing left/right and up/down arrows,

indicating the direction the cross hairs should be moved as shown in Figure 2. The yellow

rectangles provide a relative indication of the distance the base. In the example in Figure 2,

the crosshairs need to move a little to the right and a longer distance down to find the base.

5. When the base position is accurately targeted, the yellow arrows and yellow rectangles will

disappear and the valid target box will change from a red square to a green circle as shown

in Figure 3. After waiting a couple of seconds for the compass to stabilize (thats why a

Figure 2: Finding the Base

Figure 3: Capturing the base position


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tripod is so important for span height measurement), tap the green arrow to record the

span height.

6. The final step is capturing an image of the span height measurement location. Move the

ikeGPS crosshairs back up so the wire is shown in the top of the image and the base position

is shown in the bottom of the image and the entire height annotation vertical line is visible

as shown in Figure 4. After waiting a couple of seconds for the position of the annotation to

stabilize on the image, tap the green arrow to store the image. The entire height

annotation may not fit on the screen if the tripod was set too close to the wire.

Assessing the Accuracy of the Span Height Function

Field testing was performed to verify both the accuracy of the base position and the accuracy of the

mid-span height when using the ikeGPS Span Height function. For the field testing, a test site was

selected where power cables, a guy wire and cable TV conductor crossed a residential street in southern

California. The guy wire had a johnny ball insulator placed in the guy above the south side of the

street. This insulator provided a readily identifiable target and reference point on the wire. A 6

diameter orange cylinder was placed on the roadway directly under the insulator in the guy wire. Since

an extendable measuring stick was not available, the ikeGPS was used to capture a TrueSize image of

the pole, insulator and orange base marker. Using the ikeAnnotate software, the insulator height above

ground was estimated to be 27 2 as shown in Figure 5.

Figure 4: Capturing the final image


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Figure 5: ikeAnnotate Insulator Height

The ikeGPS was mounted on a tripod and placed on the opposite (north) side of the street from the

insulator to simulate actual field conditions. A typical field setup would be measuring the mid-span

clearance between the conductor and the center of the roadway, with the ikeGPS placed on the side of

the road.


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Since no precision measuring tools were available to precisely locate the position of the insulator and

the corresponding position of the base marker, the results are empirical, but should be similar to

accuracy and precision obtained when positioning an extendable measuring stick. ikeCaptures Span

Height function was used multiple times to locate the base position and to record the mid-span height.

Images of the ikeGPS screen were captured while locked onto the base position. Three of those images

are shown in Figure 6.

`

Figure 6: Base Targeting Images

In the leftmost image the crosshairs are located even with the base of the marker but need to be moved

to the left. The middle image has the crosshairs on the centered on the base of the marker, while the

crosshairs in the right most image are slightly below the marker, but the Span Height function indicatesthe crosshairs need to be moved up a little bit. From these images and other field tests, it can be

concluded that the base position is typically located to within six inches or less of the actual base

position.

Using the test configuration described above, 14 Span Height measurements were made using an

ike1000 unit. The table summarizes the results.

Span Height Summary

Using ike1000

(Actual Height 27.17 ft)

Height(ft)

Error(ft) Error %

Minimum 26.29 -0.88 -3%

Maximum 27.93 0.76 3%

Average 27.03 -0.14 -0.5%


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Several weeks later, 18 Span Height measurements were made at the same location using two different

ike300 units. This table summarizes those results.

Span Height Summary

Using ike300

(Actual Height 27.17 ft)

Height

(ft)

Error

(ft) Error %

Minimum 26.57 -0.60 -2.2%

Maximum 27.26 0.09 0.3%

Average 26.94 -0.23 -0.8%

The error of any single span height reading ranged from a minimum -3% of the span height to a

maximum of +3% of the span height using the ike1000, while the error range from -2.2% to +.3% usingthe ike300. The largest error for the ike1000 was .88 feet, while the largest error for the ike300 was .8

feet.

These variations occur for several reasons. First, the target wire (or johnny ball insulator in these tests)

is smaller than the diameter of the ikeGPS laser range finder beam. Testing indicates that the wire is

correctly targeted over .4 - .5 degrees of inclination. Second, the wire can be moving if there is a breeze

or air movement. Third, the size of the ikeGPS crosshairs and screen display resolution can result in

small changes in the inclinometer reading that are easily visible while aiming.

One way to improve the accuracy of the Span Height reading is to take several Span Height

measurements and average the result. With the ike1000, the average of the 14 span height readings

was within .14 feet (2 inches) of the actual height, indicating that the targeting and wire movement

errors are random and by recording and averaging multiple span height measurements, a more precise

span height result can be obtained.

While the ike1000 can be used for accurate Span Height readings, due to the enhanced resolution at

distances less than 300 meters, the ike300 improves on the accuracy of the ike1000.

Tips for ikeGPS Span Height Measurements

The following tips should improve the accuracy and quality Span Height measurement with the ikeGPS.

Use a tripod with a low magnetic signature. A tripod provides stability when targeting both the

wire and the base position. A low magnetic signature insures that the tripod does not influence

the ikeGPS internal 3D compass.

Wait several seconds before capturing the wire or base position. This gives the compass and

laser range finder on the ikeGPS time to stabilize and insures the most accurate readings. This is


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another reason to use a tripod, since the ikeGPS will remain in a fixed position while you

wait this short period of time.

The diagram below shows how to correctly set up and obtain mid span height

clearances.

For more informationvisit www.ikeGPS.com/contact

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