Bryan J Flynn about 1500 words116 Edison StreetWilkes Barre, PA 18702719-310-4433bjf5249@psu.edu

HORIZONTAL TIME-DEPENDANT POSITIONING (HTDP)

WHAT IT BRINGS TO THE TABLE

By: Bryan J Flynn

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Abstract

Plate tectonics is the scientific theory which explains the

motion of the Earth’s lithosphere. While tectonic undulation was

not something that was accounted for as recent as 100 years ago,

it has and will continue to be a reality which must be dealt with

in the future. In order to maintain precise work in our endeavor

to achieve accuracy we must understand what is happening below

the surface of the Earth in order to achieve success in our

working space. As surveyors, the need to recognize this

phenomenon was made apparent with the introduction of the GPS. As

technology has evolved we have been afforded the opportunity to

collect more precise data while analyzing and interpreting these

values. Over time is has been seen that based on plate tectonics

and given the data collected by means of GPS, that coordinates of

points can and will shift. For this reason in 1992 HDPT software

was developed and incorporated in surveying operations in order

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to determine current and future and in some cases past horizontal

velocity changes. This ability allows us to correct positional

coordinates and/or geodetic observations for the effect of

crustal deformation and transforming positional coordinates and

velocities between reference frames.

Horizontal Time-Dependant Position: What it Brings to the Table

How GPS changed everything

In the early 90’s and with the advent and advancement of

Global Navigation Satellite Systems (GNSS), the geodetic

community was on the verge of a revolutionary change that would

come to redefine not only the professional venues but also many

aspects of everyday life. Over the course of the following three

decades the information that is available to be both collected

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and processed is something far beyond that which the forefathers

of the surveying profession could have ever dreamed. As time

passes, it can only be expected that these advancements will

continue to redefine that which we know and are comfortable with

forcing us to be vigilant in our endeavor to remain educated and

relevant.

How this change affected the surveyor

So as the GNSS technology improved, we were able to collect

and process more data and at a much faster rate than in the past.

With that however, it became clear that over years of collecting

information on the same points, that the coordinate values

returned were not the same. In fact in some parts of the United

States, primarily the western coastal states saw significant

changes in the coordinates. Clearly over the course of time,

positions were moving and this was now something that could not

be overlooked. If the data collected on a handful of points is

changing, then it stands to reason that every point (based on the

theory of tectonic motion) is also moving. The natural phenomena

of tectonic deformation and tectonic motion were determined to be

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the primary causes behind this and therefore the need to

understand what was happening below the surface of the Earth

became apparent.

Need to understand plate tectonics

The number of plates is somewhat a matter of opinion according

to many scientists and geologists. However, according to the

Smithsonian Museum of Natural History, there are currently 8

major plates and 12 minor plates in the world and are classified

by boundary types. Divergent boundaries exist where new crust is

generated as the plates pull away from each other, while

Convergent boundaries are where crust is destroyed as one plate

dives under another. Transform boundaries occur where crust is

neither produced nor destroyed as the plates slide horizontally

past each other. Plate boundary zones are broad belts in which

boundaries are not well defined and the effects of plate

interaction are unclear.

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Figure 1 Boundary Zones

Though the number of plates varies from source to sources, the

need to understand the implications of the movement associated

with their motion is constant. A primary reason for the study

was due to the changes seen after earthquakes in the Pacific

Northwest and interconnected coastal regions. Specifically, the

largest landmass transform fault in the world today, the San

Andres Fault.

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Velocity Vectors and what they mean to us

Figure 2 Horizontal Velocity Vectors

As these faults clash together, slide alongside one another or

pull apart, the magnitude of these occurrences cause displacement

of the crust as part of the lithosphere. This natural phenomenon

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is a concern to the geodetic community for a variety of reasons,

but a major concern is the location of control. Projects, as we

all know, must have control in order for precise work to be

conducted. If the location of the ground control is

questionable, then the quality of the work associated with that

control and all subsequent information collected is also

questionable.

Capabilities of HTDP software

With the understanding of how these velocities vectors were

changing, in 1992, NOAA’s National Geodetic Survey (NGS)

introduced the Horizontal Time-Dependent Positioning (HTDP)

software. HTDP allows users to transform positional coordinates

across time and between spatial reference frames. In particular,

the current version of HTDP enables users to perform each of the

following functions: Estimate interseismic horizontal crustal

velocities, estimate the horizontal crustal displacements between

two dates, transform positional coordinates from one reference

frame to another and/or from one date to another, transform

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certain types of geodetic observations from one reference frame

to another and/or from one date to another, and transform crustal

velocities from one reference frame to another (Snay 2013).

This was and is still part of the geodetic toolkit available on

the NGS website for public use. The software over the course of

the years has been upgraded and those newer versions have been

introduced periodically just as many operating systems upgrades

based on the newest crustal motion model. These new releases

were due to improved capabilities that were consistent with the

technological advancements. One of the main changes that have

been made since the inception of the program was the corrections

for major tectonic undulation due to severe earthquakes. The most

recent, version 3.2 released in August 2012, introduced an

improved model for estimating interseismic horizontal velocities

in Alaska and a model for estimating the post-seismic motion

associated with the M7.9 Denali Fault earthquake that occurred

in central Alaska on November 3, 2002 (Richard A. Snay 2013).

Values for the model parameters that predict the velocity at any

point within the domain were estimated from 4,890 horizontal

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velocity vectors _derived from repeated geodetic observations_,

170 fault slip rates, and 258 fault slip vector azimuths (Pearson

2010).

The advancements allow more data to be process not only in

CONUS but also in the Alaskan region and more accurate

information regarding horizontal velocity vectors mainly in the

western states. This along with capability to utilize the 14-

parameter transformation for converting between reference frames

makes this version of HTDP an outstanding tool for the geodetic

community (Snay 2013). HTDP supports five fundamental functions

which include: estimating horizontal velocities, estimating

horizontal displacements between two dates, updating positions

and/or observations to a specified date, transforming positions

between reference frames and/or dates, and transforming

velocities between reference frames.

According to Snay (2003) horizontal velocity is related to

the horizontal movement on the earth’s surface caused by tectonic

plate movement and periodic displacements associated with

earthquakes. This associated crustal motion can cause what is

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viewed as static points on the surface of the earth, such as ground

control points, to move at predictable rates over time. Estimating

horizontal velocities serves the purpose of prediction of a point

in a given time in the future as well as in the past. The image

below was based on the information obtained using the

displacement prediction feature of the HTDP software.

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Figure 3 Displacement Prediction

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Figure 4 Displacement over 50 years given a 50 point file

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The International Earth Rotation Service (IERS) in the 1990s

having been armed with the knowledge of how the continual motion

of the Earth’s surface developed the ITRF in which each tectonic

plate, including the North American plate, moves continuously

(Snay 1999). Thus, if a point is collected using the NAD83

reference frame, the HTDP user applications can, given the input

parameters, provide the same points’ value in the ITRF. The same

can be said for transferring point files from one reference frame

to another. For instance from a point collected using ITRF2008 to

that points same value in NAD83 or similarly from NAD83 to WGS84.

Summary

It is clear that the longer that an area is studied, or a topic

researched, the more undesirable but sometimes beneficial the

information we find. This proverbial rabbit hole seemingly has no

end. The further we look, the more sources of error we find. As

professionals it is important that we understand phenomena such

is tectonic motion and continue to be vigilant in our data

analysis. HTDP is just one of many tools available to the

geodetic professionals and is an invaluable instrument used to

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understand the implications of the tectonic motion and its

associated horizontal velocity vectors. As a benefactor and a

contributor to the geodetic community it important that we

continue to provide the most accurate information possible so as

to not only provide potential clients with top notch products,

but also to assist in the continued advancement of geodetic

tradecrafts.

REFERENCES

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Biediger, Louise Mathews and Joan. "The Impact of Horizontal Velocity on the AFO Ground Control Database." USDA. February 2, 2012. https://www.fsa.usda.gov/Internet/FSA_File/hv_paper_final.pdf (accessed November 6, 2014).

Pearson, Chris, and Richard Snay. "Coping with Tectonic Motion." Professional Surveyor Magazine, November 2010.

Richard A. Snay, Jeffrey T. Freymueller, Chris Pearson. "Crustal Motion Models Developed for Version 3.2 of the Horizontal." Geodesy, 2013: 173-190.

Snay, Chris Pearson and Richard. "Introducing HTDP3.1 to transform coordintates across time and spatial reference frames."GPS Solution, 2012: 15.

Snay, Richard. "Part 5: Horizontal Time-Dependent Positioning." Professional Surveyor Magazine, 2003.