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Author Topic: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?  (Read 5428 times)

greenfrog5

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Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« on: January 28, 2011, 09:51:31 PM »
Thank you to all who helped me get the complex series of steps that is Garmin-Map-Making working on my Mac/PC.  I have graduated from the Map Making Support Forum to this General Discussion one.

I have taken maps of local parks and visually aligned them in Google Earth.  I traced the trails as tracks/waypoints and generated Garmin-maps for immediate use.  I have then tracked and waypointed some parts with my GPSr.  I then use 3 waypoints to re-calibrate the park map.

The first map I did, my initial 'Rough' version was generally within 30ft - surprisingly good.  I have done a second one, but haven't tracked/waypointed it yet with the GPSr.  I'm wondering what the best way is to merge/update/refine these trails, ultimately to have them added to the My Trails maps?

I have an eTrex HCx, and even with the most frequent track point setting, the tracks are somewhat jagged.  Also, I'd like to avoid retracing the trails on the corrected map if possible.  I haven't tried GPS Trackmaker yet.

Some Specific Questions:
When I'm tracing in Google Earth, I often can see the trail in the Google imagery.  How accurate is this?  Obviously I'm aligning the map to it, so adjusting my trail from what the map says to match Google Earth is probably good, right?  Is the final geoTIFF significantly more accurate?

How accurate are my GPSr-tracks vs Google Earth?  They are close to both the aerial imagery, and my original rough-tracks (immediate use map).  I know the GPSr is +/- 30ft or so.

How should I take this data and improve its accuracy for a distributed map like My Trails:
Original traced trails (immediate use map)
Google Earth imagery
GPSr tracks & waypoints (sometime multiple)
GPSr-waypoint calibrated raster maps

What is considered the best way of refining GPS maps?

Thanks
Aaron

jbensman

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Re: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« Reply #1 on: January 29, 2011, 07:16:19 AM »
Aron,

You did not ask the most important question:  What is the limitation of the img format?  You should notice the tracks for creating the map do not perfectly match the map.  That is because there are resolution limitations of the img file.

So when I GPS something, I set my GPS to Automatic and most often.  That generally provides the best result.  Lost of the trails I have in My Trails are government GPSed with points every 25 feet.  These trails generally look rough in My Trails because they are much greater resolution than the map can handle. 

I trace stuff in GoogleEarth all the time.  I traced a bunch of stuff yesterday in CO and AL that will be added to the next version of My Trails.  Tracing stuff in Google Earth works fine if you can clearly see what you are looking for.  However, sometimes the photos are not aligned properly.  So the first thing I do is find an intersection in MapSource and cut and past its cords into GE.  Then I use the history function to see which photos are aligned properly.  When the photos are properly aligned, they give you tracks that are as accurate as the resolution of the img file.

After I GPS something, I download it into MapSource.  Then I will either use the map tools (mostly erase) to clean up the track or draw it with the draw track tool.  What I need is a clean single track where the trail is.  A raw track usually will not have this-sometimes you go off the trail, go on the trail twice, gets extra points when standing still, etc.  This needs to be cleaned up.  Sometimes it is easier to just trace the trail than clean it up.  Sometimes I do a combination. 

Generally GPS is the best.  Just use your judgment on which is the best.  Yesterday I was taking trails from Trail Co Op and opening them in Google Earth.  Some I could not see the trails.  Others I could and some of them were off.  So for them, I did not use the track from Trail Co Op, I traced the exact location in GE.  Everything in My Trails is not perfect.  Its just the best data I can find and I am always trying to improve it.

I am off to go GPS some trails I traced from DNR maps and included in the last version of My Trails.  I will send you my raw track and how I clean it up before adding it to My Trails so you can see how I go about doing it. 

greenfrog5

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Re: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« Reply #2 on: January 29, 2011, 09:23:39 AM »
Thanks, very helpful.  Is there anything else to know about the limited resolution of the IMG format (aside from 'it is limited')?  Specific things to do to optimize it?

Another question: is there a good way to tell what trails I have put into my map are already represented in other maps like My Trails?  I know MapSource can't do it, is the best way to take my traced/GPSed tracks (before converting them into Maps) into MapSource so I can cycle through my maps and see what overlaps?

How do you go about de-duplicating trail data?

As I mentioned, I'm planning to regularly send you new data for My Trails from around the Bay Area, CA.

greenfrog5

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Re: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« Reply #3 on: January 29, 2011, 10:08:52 AM »
Another follow-up questions while I'm at it.

Is there a way to 'shift' (translate and/or rotate) a track that I have traced in GE?  This would be to correct a traced track to match the map's updated GPS-ed position once I can properly calibrate it with some waypoints.  Or, would the corrected trails need to be re-traced using the correct map position?

Oh, something I just thought of:  The USGS Topo plugin for Google Earth places aligned USGS quads as overlays.  Are these likely to be more accurate than the Google aerial images?  At least worth using them to double-check overall alignment issues.

Thanks, this map-making stuff is really awesome.
Aaron
« Last Edit: January 29, 2011, 10:24:27 AM by greenfrog5 »

Boyd

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Re: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« Reply #4 on: January 29, 2011, 02:06:41 PM »
You did not ask the most important question:  What is the limitation of the img format?

You might find this thread interesting: http://forums.gpsfiledepot.com/index.php/topic,1335.0.html

I think the limit may be around 5 meters (~16 feet), but it will first rear its ugly head when you create the .mp file - before compiling an .img file. The issue is that both .mp and .img files use 24 bit coordinates. Maybe someone smarter than me can do the math to determine what the maximum resolution will be?
« Last Edit: January 29, 2011, 02:10:22 PM by Boyd »

greenfrog5

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Re: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« Reply #5 on: January 30, 2011, 09:46:06 AM »
Thanks for the link.  My eTrex has WAAS capabilities.  Should I be enabling this when tracking trails for mapping purposes?  (Even though they'll get rounded by the file format)?

jbensman

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Re: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« Reply #6 on: January 30, 2011, 10:29:25 AM »
Before I answer your questions, let me show you what I did yesterday.  I started out with the IL DNR map that had trails drawn on a USGS 24K map.  I'd been to the area before and I knew the locations were just IDNR's educated guess where the trails were. Since I was planning a trip there, I did not take the time to align the map in ExpertGPS.  I just drew freehand where they said the trails were-so it was not exactly where they said the trail was.  I then looked at the area in GoogleEarth to see if I could see any of the trails.  One trail was clearly visible so I traced it and adjusted my previous tracks.  Then I added the tracks to My Trails.  This is what is in the current version of My Trails. 

Yesterday, I went to the area and hiked about 8 miles.  The trails were way off but they were very useful.  I did not hike on the trail that I traced in GE, but it was exactly where it showed connecting to a trail I was on. 

I set the tracking to most often and automatic.  I always have WAAS enabled and try to hold the GPS to get good reception.  I have a 550T because I can take a georeferenced picture.  Whenever I see something I want to add to the map, I take a picture.  I can then open the picture in MapSource (you have to use BaseCamp to download to then export as a MapSource file), look at the picture, and then name it to add to the map.  Without a camera, you can name the waypoint when you mark something you want to add.  I also take a picture of signs at trail junctions in case I need them to figure stuff out.  It is also good to stop briefly at the junction to make sure the tracklog records a point there.

Iíve attached a MapSource file that has the raw track along with the tracks and waypoints I used to add to the next version of My Trails.  For the places I hiked twice and had a double track, I used the track tool to draw a track trying to average the two tracks.  For the parts I only hiked once, I used the cut track tool to separate that part of the track.  I then named the track the trail name and cleaned it up.  When you stop you get extra points that need to be removed and if you go off the trail, that needs to be removed too.  You can use the erase tool to erase the points.  Or you can open the track by double clicking on it.  Check Center Map on selected item.  Then you can scroll through the track points and delete unwanted points as you scroll through them.  I will generally delete any point not at least 15 feet from the previous point.  When I have more than one segment for a single trail, I will use the join track function to obtain a single track for the trail.

jbensman

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Re: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« Reply #7 on: January 30, 2011, 10:47:29 AM »
Another question: is there a good way to tell what trails I have put into my map are already represented in other maps like My Trails?  I know MapSource can't do it, is the best way to take my traced/GPSed tracks (before converting them into Maps) into MapSource so I can cycle through my maps and see what overlaps?

Yes take your tracks and open in MapSource

How do you go about de-duplicating trail data?

Its a judgment call.  If someone send me GPSed tracks that conflict with existing data, I go with the GPSed.

If I have conflicting data from various sources, I will look at the area in GoogleEarth to see if I can tell if one is correct.  I consider the source.  I will also look at it on topo maps to see which makes sense.  Lots of government data comes in shapefiles and the shapefile will usually have information about the accuracy of the data.  You can also judge data by how close the points of the track are.  Some are obviously just a guess.

jbensman

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Re: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« Reply #8 on: January 30, 2011, 10:51:55 AM »
Is there a way to 'shift' (translate and/or rotate) a track that I have traced in GE?  This would be to correct a traced track to match the map's updated GPS-ed position once I can properly calibrate it with some waypoints.  Or, would the corrected trails need to be re-traced using the correct map position?

Retrace-but if it is slightly off I would not worry about it.

Oh, something I just thought of:  The USGS Topo plugin for Google Earth places aligned USGS quads as overlays.  Are these likely to be more accurate than the Google aerial images?  At least worth using them to double-check overall alignment issues.

How do you get the USGS plu in?  USGS 24K trail data can be way off.  But it can be useful

greenfrog5

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Re: Accuracy: GPS Tracks vs GoogleEarth vs GPS-Aligned TIFF?
« Reply #9 on: January 30, 2011, 12:54:25 PM »
Here is a link.  It lays a grid down and you click/download each individual map as you need them from within GE.  I haven't check on the alignment accuracy much, but the ones in my area seem well-aligned enough.

http://www.gelib.com/usgs-topographic-maps-2.htm