How GPS Works
- Fixed orbiting Satellites, with super accurate clocks, send a repeating weak data stream containing satellite ID signals and a time stamp.
- Earth based devices use the subtle differences from each signal to work out where the satellites are in relation to an imaginary sphere that aligns closely with the earth.
- Trees, buildings and cars bounce signals, clouds and the atmosphere delay the signals, and other effects cause your GPS calculated to "drift" around its trigonometry calculated location.
| 1 | Set all of the GPS devices to Decimal Degrees - DD.DDDDD using WGS84 as the datum. This works world wide, even in Antarctica. | ||||||||||||||
| 2 | Capture and store all your data in a spreadsheet as Decimal Degrees to 5 or 6 decimal places depending on how accurate you need your data to be. You can do this manually if you have only a few places, via a note paper in the field, or you can capture "Waypoints" or bookmarks on the device and translate them later, or extract via bluetooth or cable. Five DP allows you to capture data to 1.1m resolution which is ideal for most hand held GPS, which in normal conditions will provide 3-5m accuracy at most. 6 decimal places will give your data a resolution of 11cm depending on the GPS Unit quality - 7 DP is only useful if you have a high performance GPS device - otherwise its 2 more digits that you will use and possibly introduces false confidence and mistakes through human error. Resolution should not be confused with Accuracy. More info on http://en.wikipedia.org/wiki/Decimal_degrees | ||||||||||||||
| 3 | Store all of you GPS features/asset data as 2 fields in your databases, numeric, 00.00000 and 00.00000.
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| 4 | If you want to validate the data collected, use bounding fields covering the country such as must be more than 25 and less than 40 for N, and E must be greater than 50 and less than 80 or something like this (Afghanistan). For a small study area you can be more restructive. Compare the values above and you can see the difference between Sudan DD.DDDDD and Afghanistan DD.DDDDD references. If your sponsor requires other formats, then provide those using formula in Excel or batch online processing, there a numerous free online sites that will provide this as a free batch service. Do not store these translated values in your database. MGRS for example - as a civilian organisation you have no business using this data type. Do not store your data using this format. |
Degrees, Minutes and Seconds Degrees, Minutes and Seconds provide almost no value to anyone except geeks in which case ask them to explain why they are causing such pain. They are difficult for normal people to use, difficult to find on a map, and being based on Imperial 60 base, most people can’t understand the divide by/multiply by 60 translation formulas. GIS and Map programs have difficulty reading this format as it needs to be stored as a text value, also making data quality difficult to monitor and you cant apply the range bounding box QA checker described above. Don’t put both data into the same field, use two as per below.
What do you do with the Data
To capture the data you will want to store information in a series of table headings in an Excel which can be used by any GIS technician to load into a database or map. Standardise your fields where possible, eg:
| NAME DD.DDDDD DD.DDDDD DATE BY |
| Mosque 15.48064 32.50142 04JUL11 Eng. Suliman |
Then You can Make a Map
Any GIS software should be able to import from an Excel sheet, or a saved as CSV file and use the coordinates to plot the locations as sites on a map, using any other fields as labels and symbology qualifiers. This short guide was originally written late one night for a colleague based near Kandahar for very basic starter data capture in a difficult environment.
muito bom.
ReplyDeleteDuane Wilkins is a premium Project Manager with large experience in GIS.
ReplyDeletecareful with WGS84 datum....... for precise applications in no useful anymore
ReplyDeleteAP
Hi Ana, I agree that a local datum would be much more accurate, the problem is that in post-conflict environments, they don’t usually have a local datum or knowledge of it, that can be implemented at the local level, with a villager + Garmin GPS for example it wont be in the menu. In most cases sub metre accuracy would be useful to have however the cost is prohibitive for many NGOs to purchase greater than off the shelf GPS and many are untrained in how GPS works. These consumer units and smart phones are what the spatial data of Afghanistan are being built on today quite literally. But, if you are found in the countryside with a GPS unit, you are working for the infidels and therefore surplus to requirements, and this will not change for years.
ReplyDeleteMy irritation is that maps in these environments are also printed in WGS84 instead of any other useful projection, we used Web Mercator variants (as I think it has become the most printed standard worldwide by international aid agencies) so they could align with google maps. For example, you would be very surprised at how many NGO AID agencies store their data in Military Grid Reference simply because Military Maps are the only maps available to them in the field, the government in Iraq and Afghanistan don’t yet have the ability to print their own topographical maps, so Russian and USA MGRS are prevalent (eg, most data is stored in Lat Long, and MGRS in Afghanistan).
There is a recent book out by Esri on Land Administration and Sustainable Development, while not dealing with GPS, there is a need there for a GPS for Developing Environments handbook too (which was the spark for the original text the post is based on).
Kind regards
Duane.