Using GPS to lay out a field

[s219]

Thanks. You're right that the compass is the weak link. The GPS gives a bearing. If you're moving at more than a slow walk it is very accurate (same accuracy as the underlying GPS), if you're not moving it's just a random number. So i look at the bearing and speed reported by the GPS. If the speed is over 1 knot I use the GPS bearing, otherwise I use the compass bearing.

What you might want to try if you need an improvement in static bearings is to use the gyro. You would use a walking GPS "course" or a known bearing to calibrate the gyro to a known direction, then from there it will give pretty repeatable rotation measurement from your known bearing. Eventually the gyro will drift and need recalibration, but normally it's good for 5-10 minutes under gentle motions (will depend on the quality of the sensor and the software in the phone). You can build in automatic calibrations using known bearing back to your DGPS base station so that the software periodically corrects out drift. Gyro can be accurate to ~0.1 degree or better and gives very smooth data, which is great for rendering AR stuff.

 

[quicksandfarmer]

What you might want to try if you need an improvement in static bearings is to use the gyro. You would use a walking GPS "course" or a known bearing to calibrate the gyro to a known direction, then from there it will give pretty repeatable rotation measurement from your known bearing. Eventually the gyro will drift and need recalibration, but normally it's good for 5-10 minutes under gentle motions (will depend on the quality of the sensor and the software in the phone). You can build in automatic calibrations using known bearing back to your DGPS base station so that the software periodically corrects out drift. Gyro can be accurate to ~0.1 degree or better and gives very smooth data, which is great for rendering AR stuff.

Thank you for an excellent tip! I've been doing basically what you're saying with the compass -- when the unit is moving, take a GPS bearing and use that the calibrate the compass so that when the unit isn't moving I can compensate the compass. But the compass just jumps around a lot. I hadn't thought about gyro but it would work much better. I'll look into it.

 

[quicksandfarmer]

When I layout a field, I like to use LIDAR and CAD...

LIDAR is pretty cutting edge, but for me CAD means, Cardboard Aided Design!

What I do is, take a LIDAR map of the field or area of my farm I want to work with, using 2 foot contours instead of 20 foot contours like most maps.

Then I make a shadow box of the area I am doing my farm planning on, so that I can work my map.

Because of the thickness of cardboard, this scales quite nicely into two foot contours, so as I cut and layer the cardboard, I get very accurate contours of the field because each layer represents 2 feet of elevation. This unfortunately gives me "steps". To fill those in, I then use drywall compound to make the final layer very smooth, so that I get all the details of the contours, hummocks, dips, ledge outcrops, ditches, swales, etc.

Then I paint the surface for whatever I am doing. Blue for ponds, swales, streams, etc. Dark green for forest, light green for fields. I will even add in railroad modeling stuff such as ballast to show where the roads are, haybales to show where erosion control goes, and even trees. When I am done, I have a very accurate 3D model of a field or portion of my farm.

This really helps when working with Federal or State Soil Engineers who are funding various farm projects. They can instantly see what your plan is for the field, or area of the farm. Then they can approve it or not. Taking a few hours to make a scale #D model, and then having it readily approved because they can see it, and you show your devotion from the start, goes a long way to getting funded for Federal and State Grants.

Very interesting. Where do you get the underlying LIDAR data?

 

[quicksandfarmer]

At $800 the price is right. What are you using to interface with the GPS, phone, IPad? I can稚 remember the exact cost of the last Trimble system we got at work but somewhere in the $50k range and we already had a $6k data collector.

I'm using an Android smartphone. The rover has a USB port that plugs into the phone. The rover produces a stream of text ("NMEA sentences"). Here's what it might look like:

$GNGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47

where:
123519 = Fix taken at 12:35:19 UTC
4807.038,N = Latitude 48 deg 07.038' N
01131.000,E =Longitude 11 deg 31.000' E
545.4,M = Altitude, Meters, above mean sea level

The other numbers are facts about the precision of the reading.

I have an app which I wrote running on the smartphone which reads the USB port, when it receives a sentence it reads it and updates the current position. There is another sentence that gives speed and bearing.

What are you using for a coordinate system, I normally work in state plane. There are a lot of known points out there. They usually list lat. and long, UTM, and state plane coordinates. Google NGS, National Geodetic Survey and look for data sheets. I almost always tried to start a job using these known points to start from. There might be one near your job site. I知 not sure what kind of radio range you get.

Everything is based off of the base station. There are two ways you can set the base. You can put it at a known location and input the lat/long. Or you can do what they call "survey in" where you just have it use regular GPS to calculate its position. If you let it sit long enough the theory is the errors will cancel each other out. For field layout I don't care so much about absolute position as the relative size and shape, so I set the base to do a five minute survey-in. Then I walk the perimeter and make sure there aren't any obstacles. If I need to move the field I'll adjust the way it's defined on the web page, I can move that in one foot increments.

 

[npalen]

Unable to bring up the attachment in post #2.

 

[dodge man]

I wonder if there are any apps that would read that string and give useful data. Any pics of your system.

 

[quicksandfarmer]

Unable to bring up the attachment in post #2.

I'm going to try attaching it again. I consider myself a pretty clever guy but I can't for the life of me figure out how the TBN picture attachment thing works. I'm reminded of what Ben Franklin said about electricity, "it makes a vain man humble."

 

[dodge man]

I clicked on the link to look at a pic of the equipment. It looks like it is just the guts of a GPS receiver. I saw where you mentioned putting a metal tray under them for reflections, which is called multi path. Surveying equipment has that stuff built into the housings and the software also helps in rejecting multi path.

Does your equipment just track American satellites? I am not sure how many are floating around up there now. Gps(USA), European (galelio), Japanese, Chinese (Bediou sp?) and Russian (Glonass). I first started using GPS about 1993 and at that time there were long periods of the day you could not work because there were not enough satellites. With RTK you need 5 to get a fixed solution and 4 to maintain it, that assumes no obstructions. These days it not uncommon to track 15 to 20 satellites.

Do you have this in some kind of housing? Are you using some kind of pole? Kudos to you to get it working, that is way beyond me.

 

[quicksandfarmer]

I wonder if there are any apps that would read that string and give useful data.

The format of the string is NMEA which stands for National Marine Electronics Association, it's a standard for GPS. Almost any GPS device is going to use NMEA.

Any pics of your system.

The base is on the left and the rover is on the right. The big metal disk is an aluminum pizza pan, the black metal square in the center of the pan is the GPS antenna. The plastic box with the antenna sticking out of the top is the DGPS kit. On the base unit the white box is a battery.

The rover is rigged up around a paint marking wand. I attached a couple of pieces of aluminum bar to the wand, and the dish and antenna are on one end of the bar. On the other end of the bar I have the DGPS receiver on the right and the smartphone on the left. The smartphone has velcro on the back to keep its position fixed relative to the antenna.

 

[dodge man]

I tell you that’s a heck of a setup. I assume the black box in the center is the GPS antenna? One thing that will help is to get that setup in the air. We usually use a 2 meter pole to get it up over our heads and the base sets on a tripod about 5.5 feet high.