PLOTTING SURVEYING DATA IN GOOGLE EARTH

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Jul 14, 2012 (4 years and 11 months ago)

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Plotting Surveying Data in Google Earth


1


PLOTTING SURVEYING D
ATA IN GOOGLE EARTH


D
M

STILLMAN



Abstract

Detail surveys
measured with

a total station use local coordinate systems. To make
the data obtained from such surveys compatible with Google Earth, a virtual globe
that

uses
global coordinat
e system WGS84, a geo
detic transformation
is required
.
Th
e

paper
discusses

the
different coordinate systems used within Great Britain and
those used
by Google Earth. The paper
goes on to describe the computer program
developed to
generate

KML documents
fro
m surveying data, so that it can be
visualised

within
Google Earth.
It also describes accuracy tests carried out to
assess

the accuracy of the aerial imagery used
within Google Earth as well as

the
accuracy
of the
Helmert transformation

used by the program
.



Introduction

Google Earth is a free computer program which uses a virtual globe to map the Earth
using superimposed satellite and aerial imagery. Since Google Earth produces
imagery which represents the whole world, it uses global GPS coordinate system

WGS84. There are many tools which currently exist for viewing GPS data within
Google Earth. However, prior to this
project

there was no way of easily converting
data obtained from land surveying, using local coordinate system OSGB36, for
visualisation wit
hin Google Earth. Doing so will allow the surveyor to view their data
in relation to the imagery and terrain and will also enable the surveyor to quickly
check that all necessary data has been captured.



Background

There are four main areas that need to
be considered, they are land surveying,
coordinate systems,

conversions and
geodetic

transformations
,

and Google Earth.


Land Surveying

Total stations measure horizontal angles, vertical angles and distances from a single
set up
.

Detail Surveys use total s
tations to measure all of the detail from a control
framework required to produce a plan or map.

Once a control framework is set up,
the total station
can be set up at

a control station and detail can be measured by
setting
a prism on a detail point. Once
all of the detail has been measured, the raw
data file from the total station can be
reformatted into the

more readable and
standard form of a field file. This can be done using software such as LISCAD
(2009). The field file can then be edited to insert in
formation about the control
stations the detail points were measured from. This new file is called an ‘edited field
file’, and can be used to find the coordinates of all of the detail points.


Coordinate Systems

The coordinate system traditionally used in
Great Britain is OSGB36

(Ordnance
Survey Great Britain 1936)
, a two
-
dimensional ellipsoidal coordinate system based
on the Airy 1830 ellipsoid. The Airy 1830 ellipsoid is a simplified shape of the Earth
chosen as it fits the Earth particularly well for the

region. The Transverse Mercator
Plotting Surveying Data in Google Earth


2


map projection is commonly used with OSGB36 to give coordinates in Eastings and
Northings instead of latitude and longitude. Heights used in Great Britain are based
on the Ordnance Datum Newlyn (ODN). This is a one
-
dime
nsio
nal coordinate
system. The heights are geoid heights


heights relative to ‘mean sea level’, which is
a tide
-
gauge in Newlyn.


Google Earth uses GPS coordinate system WGS84 (World Ge
o
detic System 1984)
for latitude and longitude, and Geoid model EGM96 (Ear
th Gravitational Model 1996)
for heights. WGS84 is a coordinate system based on the GRS80 (Geodetic
Reference System 1980) ellipsoid, an ellipsoid which fits the whole Earth the best.


Coordinate systems
can be

known as datums or Terrestrial Reference Syst
ems
(TRSs) when they have been fixed to the Earth by a datum definition. Terrestrial
Reference Frames (TRFs) are set up to realise the datums so that they can be used
in the real world. For example
,

OSGB36 traditionally used trig pillars

on hill tops as
po
ints of known coordinates
, enabling new points to be derived by measuring from
the pillars.


Conversions and
Geodetic Transformations

Coordinate conversions are different to transformations in that they do not involve a
change of datum (Iliffe and Lott, 20
08). Examples of conversions are going from
ellipsoidal coordinates to Cartesian coordinates, or to map coordinates using a
projection. The parameter values for the conversion are defined and as such they do
not lose accuracy and are reversible to get the
same answer. Transformations on the
other hand are used to transform coordinates between different datum realisations
and will be affected by the surveying imperfections of both coordinate reference
systems.


Two ellipsoidal datums can differ in position o
f the origin of coordinates, in the
orientation of coordinate axes, and in the ellipsoid size and shape. The ellipsoid size
and shape can be eliminated most simply by converting to three
-
dimensional
Cartesian coordinates. A Helmert datum transformation can

be

applied to a TRF to
rotate the Cartesian axes, translate the origin and alter the scale.

The Helmert datum
transformation does not take into account regional distortions in the TRFs, and as
such using it to transform between OSGB36 and WGS84 can give e
rrors of up to 4m
(Ordnance Survey, 2007). Local transformation parameters can be used for a more
accurate Helmert transformation. The Ordnance Survey has developed a more
complicated transformation known as OSTN02 which takes into account the variable
loc
alised distortion. The OSTN02 transformation consists of grid translation vectors
which cover the country at a 1km resolution. Bi
-
linear interpolation is used on the
grid of translation vectors to calculate a shift corresponding to the local distortion.


G
oogle Earth

Google Earth
(2009)
is a virtual globe, map and geographic information program. It is
a freely available program that superimposes imagery obtained from satellite and
aerial photographs onto a 3D model of the world.


The user

s geographic data
can be represented easily on Google Earth through the
use of Keyhole Markup Language (KML)
documents. These documents

can be used
to show points, paths, polygons and ground overlays.

Plotting Surveying Data in Google Earth


3


The v
ertical aerial photographs
used in Google Earth have been
georeferen
ced

to
align with the coordinate system
. The process of georeferencing involves
identifying
ground control points in the image for which accurate coordinates are available. A
transformation is then calculated by computer software which processes the image
so that it aligns to the ground coordinate system (Wolf and Dewitt, 2000). Mosaics
are used to stitch many aerial photographs together. Controlled mosaics use rectified
photos so that all of the photos are vertical and at the same scale. In mosaic
assembly
, image positions of common features in adjacent photos are matched as
closely as possible. A plot of control points is used to match and constrain positions,
similar to the technique used in georeferencing. Uncontrolled mosaics simply match
the image deta
ils of adjacent photos without using the ground control, which is
quicker but less accurate in terms of the coordinate reference system.
Semicontrolled mosaics have either no ground control or use photos that have not
been rectified.



Computer Programming

The computer program was developed using Java, a modern object
-
oriented
programming language.
Java is portable
, meaning programs developed
with

the
Java programming
language can be run on any
thing

that

supports the Java platform.
Java is supported by

all
major
PC
operating systems,
as well as many web
browsers, mobile internet devices and mobile phones (Flanagan, 2005).


The open
-
source integrated programming environment Eclipse (2009) was used for
the development of the program.


The program, named Google

Earth Plotter, was chosen to use an approximate
Helmert datum transformation. The steps required to change a set of land surveying
coordinates (OSGB36 Easting and Northing and ODN height) to Google Earth
coordinates (WGS84 latitude and longitude and EGM96

height) are as follows:


(a)

Convert grid Eastings and Northings to ellipsoidal latitude and longitude using
map projection formulae;

(b)

Convert ellipsoidal latitude, longitude and height to 3D Cartesian coordinates;

(c)

Use the Helmert datum transformation to trans
form between OSGB36 and
WGS84 TRFs;

(d)

Convert 3D Cartesian coordinates back to ellipsoidal latitude, longitude and
height;

(e)

Discard WGS84 ellipsoid height. Use approximate Geoid height found by
adding
the EGM96
-
ODN
height difference (0.8m) to ODN height.


The

computer prog
ram has three main functions. The first is to

plot a single point,
entered directly into

the user interface by the user. The second function
plots

multiple points from a file
,
and
the third
plot
s

an edited field file.

All of the functions
use

the input data to create a KML file which includes the new WGS84 coordinates
and formatting details. Google Earth uses the KML files to superimpose the data on
top of
its

aerial imagery.


Figure 1 shows the interface of the
program for plotting a point an
d for plotting field
data. Figure 2 shows field data plotted on Google Earth.

Plotting Surveying Data in Google Earth


4





Figure 1.
The
Point and
Field Data tab
s

of Google Earth Plotter





Figure 2.
KML representing field data collected during

the s
urveying field course at
Loughbrough Univers
ity




Plotting Surveying Data in Google Earth


5


To plot field data, the computer program has to convert detail points into coordinates
using

the horizontal angles, vertical angles and slope distances in the edited field
file.
Then the coordinates are transformed using the steps above.
The control

stations are plotted as triangles,
with

a thick white path join
ing
the stations of the
control framework

(Figure 2)
. The control stations
are coloured differently
. The detail
points are plotted as circles, with their colour corresponding to the control st
ation
they were measured from. The point IDs can also be shown

as labels for the points
.
The user can
specify

the vis
ibility of the labels

with

the program. They can be
shown
always, only when highlighted by the cursor, or not at all. The user also has the

option to join consecutive points. This creates paths between points which have
consecutive point IDs. The paths are coloured according to the control station
used
to measure the

points.

The paths are useful to view the order
in which

the points
were meas
ured. This can be useful to see what the points represent. For example,
when points are measured along the side of a road
,

the path will
represent

the edge
of the road. This gives the KML a more map
-
like quality.


The user interface (Figure 1) also contain
s text boxes for the inputs of positional
corrections. These corrections are used to align the KML with the Google Earth
imagery. The corrections, in the form of East, North and vertical corrections, are
simply added to the OSGB36 coordinates before the tr
ansformation is calculated.
The user can find out
the corrections required

to align the KML with the imagery by
measuring uncorrected KML with the Google Earth ruler tool.



Accuracy Tests

The a
ccuracy tests made use of the Ordnance Survey’s published coor
dinates for
the
passive stations of the National GPS Network. The coordinates are given in
OSGB36 and WGS84
.

T
he stations used were pillars, as they
can

be located in the
Google Earth
imagery. The computer program was used to generate KML from the
OSGB36 c
oordinates. The true WGS84 coordinates and the coordinates of the
stations in the Google Earth imagery were added to the KML. This allowed the three
different coordinate types to be compared. The Google Earth ruler tool was used to
find the distance betwee
n the true (published) coordinates of the stations and the
coordinates from the Helmert transformation and Google Earth imagery.
The results
of the accuracy tests are shown in Table 1. The averages are based on a sample of
17 passive stations. Two of the
1
7
stations were unable to be located in the imagery
due to poor resolution.



Distance Between

Published and
Imagery (m)

Published and
Helmert (m)

Imagery and
Helmert (m)

Average

2.7

2.1

1.3

Standard Deviation

1.3

0.8

1.3


Table 1. Results of accuracy t
ests






Plotting Surveying Data in Google Earth


6


Conclusions

The program can quickly and easily plot survey data on Google Earth. The positional
corrections
used
are only approximate as they do not take into account a rotation or
scale error. But they have been highly effective in aligning the
KML with the imagery.


The accuracy tests
show

that the Helmert transformation used by the program
is on

average 1.3m
horizontally
from the position in the imagery
. The true published
values
of the passive stations were even further from the imagery
. This
is an
error
of
position in

the
Google Earth imagery. The accuracy of the aerial imagery depends on
how well
-
aligned it is to the coordinate system used by Google’s model of the Earth.
The
alignment
accuracy
depends upon how much time and effort has gone in
to
georeferencing the vertical photographs, and the preparation of the digital mosaics.
The Helmert transformation
was found to be

2.1m from the published values. The
use of OSTN02 would eliminate this error, however, it can be seen that this will not
help

the KML
align with

the imagery.


The program can be used by a surveyor to create a KML document for data that has
been measured in the field. This can be used as a quick check to verify the captured
data
,

us
ing

the imagery to identify any missed areas. Th
e surveyor can also forward
the KML by email to show the client the work that has been undertaken.



Recommendations for Use

The edited field file needs to be formatted in a specific way for best results. There
must not be any blank lines, and IDs of detai
l points should be numerical only. This
allows the program to plot lines between consecutive detail points.



Download

The program can be downloaded at:
http://code.google.com/p/google
-
earth
-
plotter/



References

Eclipse, 2009.
About the Eclipse Foundation
.
<
http://www.eclipse.org/org/
>,
[accessed 23/04/2009].


Flanagan
, D., 2005.
Java in a Nutshell
. 5
th

ed.,
Sebastopol:
O'Reilly
.


Google Earth, 2009.
Google Earth
. <
http://earth.google.com/tour.html
>, [accessed
04/05/09].


Liscad, 2009. LISCAD Surveying & E
ngineering Software.
<
http://www.liscad.com/liscad/
>, [accessed 25/04/2009]


Ordnance Survey, 2007.
A guide to coordinate systems in Great Britain
,

Southampton: Ordnance Survey of Great Britain.

Available at:
http://www.ordnancesurvey.co.uk/


Wolf,
P.R. an
d
Dewitt, B
.A., 2000.

Elements of
P
hotogrammetry: with
A
pplications in
GIS
.

3
rd

ed., Boston
: McGraw
-
Hill
.