ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

Why your GPS is sometimes wrong

Updated on November 27, 2019

Global Positioning System

The Global Positioning System

The Global Positioning System (GPS) comes standard in a wide range of items; mobile phones and cars just to name two. They are small and relatively inexpensive (as an aside, when I was an Army surveyor in the 80s, our GPS units needed a Landrover to carry them and cost about $250, 000) and we have come to rely on them to find our way around.

But many of s will have come across that odd time when technology lets us down. The story the GPS was telling us wasn’t quite right. So how can a system with such a remarkable inherent accuracy lead us up the garden path? The truth is, it’s probably not the GPS that’s the problem, it’s the map behind it.

To better understand the relationship between the GPS and the graphic display we need to go back and look at some of the cartographic (and geodetic) basics. The following information is a very brief overview of what is going on inside your system.

The Earth’s Shape

One of the biggest problems facing anyone wanting to map the Earth is the fact that it is inconveniently not flat. Throughout history cartographers have grappled with the question of how to map such an odd shape. Over time, map makers came to recognise three basic models of the Earth.

Its true Shape While we think of the Earth as round, in reality it‘s an oblate spheroid, that is, flattened at the poles and bulging at the equator. To add to the confusion it has a very uneven surface with high mountain ranges and deep oceans; all in all, not a very good surface to lay a map grid out on.

The Geoid This is a theoretical surface that roughly corresponds with sea level. To be more precise, it is an equipotential surface where gravity and centrifugal force are constant. While it is much better, it too has its problems. Since gravity is driven by mass, where the Earth is denser, gravity is greater. Hence the geoid has dips and bumps that are difficult to model. We needed a more regular shape.

The Spheroid Sometimes called the Reference Spheroid, this is a mathematical surface created by rotating an ellipse (oval) about its north and south axis. It is defined by the length of the radii from the Earth’s centre to the poles and the equator. In the diagram, a is the semi-major axis and b is the sem-minor axis. This produces a very even and regular surface however it does depart from the true shape of the Earth enough to introduce some error.

Differing Spheroids

If all this isn’t confusing enough, if we look at maps from different parts of the world, we will find a number of different spheroids have been used. To understand why this is we need to go back to the true shape of the Earth. Basically it is like a big lumpy potato. Before we were able to map the world in one go, cartographers had to do it a piece at a time. As each continent was mapped, a spheroid was developed that best fitted the area. Picture it as mathematically jiggling the spheroid until it coincides, as best as possible, with the surface of the Earth. The variations were made to the length of the axes creating a slightly different sized model. Once the shape of the area was determined, cartographers could set about drawing maps that best represented their area of responsibility.

Finally, thanks to the advent of satellite positioning systems (there were others before GPS) we were able to develop a spheroid that was a best mean fit of the entire planet.

How does the GPS work?

To put it in very simple terms (more detail in a later hub) the GPS works by measuring the time taken for a signal to get from the satellites to the receiver. As the position of the satellite is known, a distance to the location can be calculated. By using four or more satellites, a position can be determined with great accuracy. At this point, the location is identified using Cartesian Coordinates which are X, Y and Z coordinates from the Earth’s centre. The GPS equipment then converts the position to latitude and longitude based on the spheroid selected by the user.

Sources of error

So what does all this mean to the user? Well, there’s going to be a problem if the map doesn’t match the coordinates from the GPS. In Australia, the difference between the World Geodetic System and the Australian Geodetic Datum is about 180 metres. The map itself, however, is a haven for error. After all, you’re trying to fit a big round thing on a small, flat sheet of paper (or screen). So if you do find that the GPS puts you in a place where you can clearly see you are not, the reason could be one of the following:

· The cartographer got it wrong

· Distortion from the map projection

· Detail on the ground has changed (roads, fences, new dams etc)

· The map was made from low precision source data

· Shrinkage and stretching of a paper map

· Poor printing

· The base map was not aligned correctly with the true coordinates

The information I have given you so far is a bit of a ramble. It is meant as a very brief introduction

To really get the best out of your Global Positioning System it helps to understand what a map really is and what it represents. You should know enough to be able to recognise when something is wrong.

Reference Spheroid

Comments

    0 of 8192 characters used
    Post Comment

    No comments yet.

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, hubpages.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://corp.maven.io/privacy-policy

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)
    ClickscoThis is a data management platform studying reader behavior (Privacy Policy)