You’ve bought your GPS equipment and you’re ready to start collecting data for your GIS. That’s great! But before you dive in and do something foolish, take a moment to learn how you can prepare yourself to collect good, quality GIS data you’ll be proud to hand off.
1. Understand how GPS works. How do all of those satellites up in the sky tell you your XY location? What is differential correction? Does GPS work on a cloudy day? Before you go out and try to collect GIS-ready data, educate yourself about how the GPS system works.
Knowing how the system works will help you work WITH your equipment. You will understand what the GPS settings in your field software mean and how to do basic things like turn on or off SBAS, check your DOP values, and ensure your almanac is current.
Today’s GPS equipment is easy enough for anyone to use without knowing a thing about the GPS system, but if you want to collect quality field data, educate yourself first.
2. Design a field-friendly workflow. Many GPS software packages highlight the ease with which you can take your GIS out into the field. And it’s true, it is easy. But it’s not always recommended.
Your office GIS probably has functionality that simply isn’t suited for field work. Things like attributes in your feature classes that describe the proper placement of labels, related tables that give access to esoteric information, a scale that covers hundreds of square miles, among others.
At best, much of this is simply not necessary in the field. At worse, taking a complex GIS into the field on a mobile GPS unit can result in poor performance and frustrated field workers. Unless you are using a laptop or Windows tablet in the field and don’t mind wading through more data than necessary, you will have to simplify your GIS data first.
Design a solid workflow for how you will move from office GIS to field-ready GIS and back. This may include creating a special database which is a subset of your office geodatabase, maintaining a ‘field map document’ scaled and symbolized for field work, and stripping down feature classes to include only attributes necessary in the field.
Other workflow items to consider are when and where to store back-up files, how (or if) to incorporate post-processing of your GPS data, and how to incorporate metadata to document how the GPS data were collected and their accuracy.
3. Keep your equipment well maintained. This means three things. First, keep the unit charged up and make sure you have plenty of memory. Get in the habit of clearing out old files and data from the device.
Second, keep your firmware up to date. Firmware is the files that affect the GPS receiver itself. Changes to the GPS or SBAS satellites often result in the need to download and install updates to the unit’s firmware. Failure to do so can result in an out of date system unable to connect to GPS or SBAS.
Third, keep your field software up to date. If you are paying for software maintenance, you should keep up with new releases and keep your GPS unit current. Which leads me to the next habit.
4. Know the ins and outs of your GPS equipment. I’m not suggesting that you become an expert on every single thing your field software and hardware can do, though that’s not an impossible feat. I am suggesting that you learn the essential functionality that is used in your workflow and know it well.
This means understanding how to install the software, configuring the GPS settings, using the software to collect new GPS data as well as update existing GIS features. Understand the sounds your GPS unit makes and what they mean. Know how the software logs GPS data and potential problems to watch out for.
You will also want to know how to make a Bluetooth connection to a peripheral device such as a higher accuracy GPS receiver or laser rangefinder, as well as how to perform a system reset if the unit becomes unresponsive.
Many people learn only a small portion of their field equipment and never venture beyond that narrow comprehension. Doing so limits your ability to apply more efficient and effective workflows. Get the most return on your investment by investing in some training. It will definitely pay off.
5. Practice good field technique. What does this mean? It means knowing how to hold your GPS unit so no part of your body is blocking the antenna’s view of the sky. Or letting your unit download almanac or other files it needs before you start using it to collect data. Basically, it means doing your part to collect good data.
You should know the proper procedure to log point data and for collecting lines and polygons in both vertex and streaming modes. If your field software allows it, know how to monitor the accuracy of your GPS data so you can collect the highest quality data possible.
If you are syncing your field edits with your GIS over WiFi, know the common pitfalls and how to overcome them. If you are post-processing your field data, be sure to get in the habit of transferring your field data to your office computer each day. Start a fresh GPS file each time you go out into the field, don’t try to re-use an old one, even if it’s just from yesterday.
6. Don’t rely on WAAS alone. WASS, or SBAS, is freely available and is great for providing a basic level of differential GPS in real-time. All you have to do is turn it on.
But it’s not very reliable.
Like GPS, WAAS is a line-of-sight technology which means your GPS antenna must be within line of sight of the WAAS satellite(s). Unlike GPS, however, the WAAS satellites sit very low in the sky. This means the line of sight can easily become broken by buildings, mountains, or even trees.
When that happens, you temporarily lose the WAAS corrections and any data collected will reflect it in their accuracy. You can easily end up with a dataset that has some WAAS corrected positions and some autonomous, or raw, GPS positions. In this case, your accuracies will be all over the place.
There is nothing wrong with using WAAS but just don’t rely on it for high accuracy GPS data. Supplement WAAS with post-processing or purchase a high accuracy receiver. That way you get the benefit of corrected positions out in the field when WAAS is available AND high accuracy GPS data for your GIS thanks to post-processing.
And finally, the biggest one of all…
7. Avoid datum shift like the plague! Datum shift occurs when GIS data collected in one datum are shifted into a different datum without properly accounting for the correct transformation. The result is data that are mis-aligned by up to several meters.
Datum shift is insidious when working with GPS. It has a way of creeping in when you think everything is going smoothly. Why? There are two main reasons.
First, when you apply differential correction to your GPS data – be it real-time in the field and/or post-processing back in the office – you are changing the geometry of your GPS data to that of the source of the differential corrections.
The actual raw GPS data are coming in using WGS84 lat/long. Your field software may be projecting the GPS data from WGS84 into your chosen projection - say UTM NAD83 Z12N. If you are using WAAS, for example, the raw GPS data are shifted into the ITRF00 datum. That one happens to be only a few millimeters difference from WGS84 so any resulting datum shift would likely go unrecognized. If you post-process your data back in the office, though, the datum of the base station will dictate the datum your GPS data are shifted into.
An on it goes. What this means is that the onus is on YOU to understand how the system works. Know the situations that have the potential to introduce datum shift and make sure you apply the correct procedure to prevent it from creeping in.
The second main source of datum shift in GPS data is exporting your GPS data to your GIS. Depending on what software you are using, you will have to account for the proper datum and/or projection of your GPS data when you export it to a GIS format.
The most common mistake here stems from not understanding how to use the office software that performs this function. You must tell the software what the proper datum of your GPS data is. That may or may not be the same as which datum you’d like for your data to be in.
Make sure to take the time to educate yourself about datum shift in GPS so that you know how to avoid it – or – if it occurs, you’ll know how to fix the problem.
So there you have it – seven good habits to foster so that the GPS data you collect will be high quality.
You’ve bought your GPS equipment and you’re ready to start collecting data for your GIS. That’s great! But before you dive in and do something foolish, take a moment to learn how you can prepare yourself to collect good, quality GIS data you’ll be proud to hand off.
1. Understand how GPS works. How do all of those satellites up in the sky tell you your XY location? What is differential correction? Does GPS work on a cloudy day? Before you go out and try to collect GIS-ready data, educate yourself about how the GPS system works.
Knowing how the system works will help you work WITH your equipment. You will understand what the GPS settings in your field software mean and how to do basic things like turn on or off SBAS, check your DOP values, and ensure your almanac is current.
Today’s GPS equipment is easy enough for anyone to use without knowing a thing about the GPS system, but if you want to collect quality field data, educate yourself first.
2. Design a field-friendly workflow. Many GPS software packages highlight the ease with which you can take your GIS out into the field. And it’s true, it is easy. But it’s not always recommended.
Your office GIS probably has functionality that simply isn’t suited for field work. Things like attributes in your feature classes that describe the proper placement of labels, related tables that give access to esoteric information, a scale that covers hundreds of square miles, among others.
At best, much of this is simply not necessary in the field. At worse, taking a complex GIS into the field on a mobile GPS unit can result in poor performance and frustrated field workers. Unless you are using a laptop or Windows tablet in the field and don’t mind wading through more data than necessary, you will have to simplify your GIS data first.
Design a solid workflow for how you will move from office GIS to field-ready GIS and back. This may include creating a special database which is a subset of your office geodatabase, maintaining a ‘field map document’ scaled and symbolized for field work, and stripping down feature classes to include only attributes necessary in the field.
Other workflow items to consider are when and where to store back-up files, how (or if) to incorporate post-processing of your GPS data, and how to incorporate metadata to document how the GPS data were collected and their accuracy.
3. Keep your equipment well maintained. This means three things. First, keep the unit charged up and make sure you have plenty of memory. Get in the habit of clearing out old files and data from the device.
Second, keep your firmware up to date. Firmware is the files that affect the GPS receiver itself. Changes to the GPS or SBAS satellites often result in the need to download and install updates to the unit’s firmware. Failure to do so can result in an out of date system unable to connect to GPS or SBAS.
Third, keep your field software up to date. If you are paying for software maintenance, you should keep up with new releases and keep your GPS unit current. Which leads me to the next habit.
4. Know the ins and outs of your GPS equipment. I’m not suggesting that you become an expert on every single thing your field software and hardware can do, though that’s not an impossible feat. I am suggesting that you learn the essential functionality that is used in your workflow and know it well.
This means understanding how to install the software, configuring the GPS settings, using the software to collect new GPS data as well as update existing GIS features. Understand the sounds your GPS unit makes and what they mean. Know how the software logs GPS data and potential problems to watch out for.
You will also want to know how to make a Bluetooth connection to a peripheral device such as a higher accuracy GPS receiver or laser rangefinder, as well as how to perform a system reset if the unit becomes unresponsive.
Many people learn only a small portion of their field equipment and never venture beyond that narrow comprehension. Doing so limits your ability to apply more efficient and effective workflows. Get the most return on your investment by investing in some training. It will definitely pay off.
5. Practice good field technique. What does this mean? It means knowing how to hold your GPS unit so no part of your body is blocking the antenna’s view of the sky. Or letting your unit download almanac or other files it needs before you start using it to collect data. Basically, it means doing your part to collect good data.
You should know the proper procedure to log point data and for collecting lines and polygons in both vertex and streaming modes. If your field software allows it, know how to monitor the accuracy of your GPS data so you can collect the highest quality data possible.
If you are syncing your field edits with your GIS over WiFi, know the common pitfalls and how to overcome them. If you are post-processing your field data, be sure to get in the habit of transferring your field data to your office computer each day. Start a fresh GPS file each time you go out into the field, don’t try to re-use an old one, even if it’s just from yesterday.
6. Don’t rely on WAAS alone. WASS, or SBAS, is freely available and is great for providing a basic level of differential GPS in real-time. All you have to do is turn it on.
But it’s not very reliable.
Like GPS, WAAS is a line-of-sight technology which means your GPS antenna must be within line of sight of the WAAS satellite(s). Unlike GPS, however, the WAAS satellites sit very low in the sky. This means the line of sight can easily become broken by buildings, mountains, or even trees.
When that happens, you temporarily lose the WAAS corrections and any data collected will reflect it in their accuracy. You can easily end up with a dataset that has some WAAS corrected positions and some autonomous, or raw, GPS positions. In this case, your accuracies will be all over the place.
There is nothing wrong with using WAAS but just don’t rely on it for high accuracy GPS data. Supplement WAAS with post-processing or purchase a high accuracy receiver. That way you get the benefit of corrected positions out in the field when WAAS is available AND high accuracy GPS data for your GIS thanks to post-processing.
And finally, the biggest one of all…
7. Avoid datum shift like the plague! Datum shift occurs when GIS data collected in one datum are shifted into a different datum without properly accounting for the correct transformation. The result is data that are mis-aligned by up to several meters.
Datum shift is insidious when working with GPS. It has a way of creeping in when you think everything is going smoothly. Why? There are two main reasons.