Susan K. Ottersen – GIS Specialist Tim Kay – Collection Systems Tech

CLACKAMAS COUNTY – WATER ENIVRONMENT SERVICES

Outline of Presentation Section A: GPS

Part 1: What is GPS?

Part 2: Types of GPS units

Part 3: GPS Errors

Part 4: Collecting data

Section B: GIS

Part 1: What is GIS

Part 3: Office Application

Part 2: Why use GIS

Part 1: What is GPS? The Global Positioning System (GPS) is a satellite-

based navigation system made up of a network of 24 satellites placed into orbit by the U.S.

The satellites circle the earth 2x a day in a very precise orbit.

Each satellite transmits a very low radio signal with its location and unique identifier

1 master Control station, 5 monitor stations, multiple ground antennas

Part 1: What is GPS? Satellite Configuration

Part 1: What is GPS? Generating Location – MINIMUM OF 3 SATELLITES

Part 1: What is GPS WAAS - Wide Area Augmentation System

2 satellites that hover in stationary positions over the equator.

Communication Satellite developed by the FAA to augment GPS for air navigation

Sits in the southern direction

Real time measurements in the field to achieve better than 1 metre accuracy

Part 1: What is GPS? Differential Corrections – removes atmospheric

interferences

Either real time as with WAAS

In the office with Post-Processing software

Part 2: Types of GPS

Part 2: Types of GPS Where To Start?

Why GPS?

Inventory, Analysis, Asset Management, Aging features,

What will I be mapping?

Point, line, or area features

What are the accuracy needs

Based on the size of the smallest feature, or smallest distance between features.

How will I store and use the data

ESRI, MicroStation, Etak, Open Source programs

Part 2: Types of GPS Questions to ask?

What resources are available

User knowledge, designated employees

Time frame

How long do I have for the project

Money

Accuracy, cross uses, # of units, additional accessories, software

Part 2: Types of GPS Units Recreational ($100 - $1000)

Garmin, Delorme, Magellan, Rand McNally, TomTom

Scientific

Trimble ProXT, SXBlue II, GISMapperTM 7, TomTom, Magellan Professional MobileMapper

Survey

Leica, Nikon, Topcon ($2000 - $25k)

Part 2: Types of GPS Units Recreational Professional

Recreational receivers are designed to acquire a location fix quickly without the need for pinpoint accuracy. Hikers, bikers, hunters….. Outdoor enthusiasts, this accuracy is perfectly adequate, they can generally find what they are looking for once they get within 10 meters of it.

Professional GIS users typically require very accurate placement of features, often within a meter or better, so that data layers can be overlaid and intricate spatial relationships determined.

Part 2: Types of GPS Units Recreational Professional Many are capable of real-time

differential correction, but DO NOT have the ability to post process.

Even with the strongest, most accurate differential correction signal, recreational receivers are simply not built to achieve submeter accuracy

Higher specifications on both hardware and software in the receiver.

Ability for both real-time differential correction, and post processing

Part 2: Types of GPS Units

With the increasing pressure to do more with less, the lower price of a recreational receiver is significant, however the inability for a ‘rec’ receiver to do the job, outweighs the initial savings.

Able to download data

Programmable for your use

Part 2: Types of GPS Units To reliably achieve the submeter level of accuracy is

the professional GPS positioning is required.

Contributing factors

GPS quality control

Electromagnetic shielding

Antenna technology

Part 3: GPS Errors

Part 3: GPS Errors

Professional GPS units give users control over the quality of the position points that are collected. Through a simple interface, the user can establish specific thresholds for acceptable data quality.

These quality control settings essentially allow the user to filter out any poor data that may degrade the overall quality of the location coordinates, resulting in greater accuracy in the final dataset.

GPS Control

Part 3: GPS Errors

By their very nature, GPS signals are extremely weak and are easily degraded by interference from nearby electrical devices such as laptop computers. High end GPS products are designed with advanced shielding technology that MINIMIZES the affects of stray electromagnetic signals from other equipment.

Electromagnetic Shielding

SIGNAL OBSTRUCTION POWER LINES/TOWERS

Part 3: GPS Errors

Acquisition of quality GPS signals requires a well-tuned antenna. The antenna provided with professional grade units are designed to allow poor quality signals to be distinguished from high quality signals.

When receiving transmissions in built-up urban environments and under tree canopy, there is degradation of accuracy due to multipath signals.

Antenna Technology

SIGNAL OBSTRUCTION TREE CANOPY

Part 3: GPS Errors

GPS signals have been degraded by being reflected from buildings and other overhead features on the way to the receiver.

While the antennas on professional receivers are designed to recognize and filter out multipath signals, many ‘rec’ receivers have no multipath mitigation at all.

Antenna Technology, con’t

Part 3: GPS Errors

This means that some ‘rec’ receivers appear to track better than pro-grade receivers in difficult environments, however, the positions derived from these measurements may be so badly affected by the multipath that relying on them is potentially worse than having no position at all.

Antenna Technology, con’t

Part 3: GPS Errors Recreational units will typically collect all data, but

this ‘productivity’ can come at the expense of accuracy. And data from a ‘rec’ unit contains insufficient information to enable postprocessing.

SIGNAL OBSTRUCTION WALLS AND BUILDINGS

Part 4: Field Observations Tim REMEMBER……WHEN GPS’ING TO THINK ABOUT THE

POTENTIAL OBSTRUCTIONS AROUND YOU

Part 1: WHAT IS GIS? GIS = Geographical Information Systems

Hardware, software and data for capturing, managing, analyzing and displaying all forms of geographically referenced information

whatisgis.wmv

Part 2: Office Application Processing

SSF > .Cor > .Shp > SDE

Time consuming

Part 2: Office Application

Part 2: Office Application

Part 3: Why GIS & GPS

Part 3: Why GIS & GPS

Part 3: Why GIS & GPS Next Phase – Asset Condition

Part 4: Fini Questions?