Litchi - How Many Decimal Places Out for GPS Coordinates in .SRT?

[Lindsaybev]

After reading in the forums, I noticed that the .SRT (subtitle overlay) files between the DJI Go app and the Litchi app differ in the number of decimal places the Lat and Long. coordinates. It showed the DJI Go at 4 decimal places and the Litchi at 5 decimal places.

Can someone please confirm whether or not this is correct? Or can someone please confirm how many decimal places out the Litchi app will provide? (I read somewhere it went out 6, but have not actually seen a picture posted. What I saw was 5.)

Also, can someone confirm that I do indeed get an .SRT file from the Litchi app?

This is so much fun! I can't seem to learn fast enough!

Thank you so very much!

 

[Fly Dawg]

After reading in the forums, I noticed that the .SRT (subtitle overlay) files between the DJI Go app and the Litchi app differ in the number of decimal places the Lat and Long. coordinates. It showed the DJI Go at 4 decimal places and the Litchi at 5 decimal places.

Just for an .srt, that much detail is not needed, unless you are doing very detailed mapping. The difference is minuscule.

 

[umanbean]

I just stepped out to the backyard and tried it with GO and Litchi.

After turning on 'video captions' in GO, both recorded an SRT file... and the GPS co-ords on both are four decimal places.

 

[Lindsaybev]

Just for an .srt, that much detail is not needed, unless you are doing very detailed mapping. The difference is minuscule.

Well, for me, it is a bit important. I would want 5 decimal points out, and saw that a man had posted his substitles on his video, and that the one with Litchi had 5 decimal points out. But I just saw it in a pic he put up of the coordinates on the video. He did not comment on it in anyway. 4 digits out = approx. 36 feet, not too terribly accurate. 5 decimal points out = approx. 3.6 feet.

If it interests anyone: Measuring accuracy of latitude and longitude?

Happy flying!!

 

[Lindsaybev]

I just stepped out to the backyard and tried it with GO and Litchi.

After turning on 'video captions' in GO, both recorded an SRT file... and the GPS co-ords on both are four decimal places.

Umanbean...Udaman! lol. Thanks for doing that for me! Too gracious.

Now I have to wonder about this other post, and how he got his information. Ha. Always one more step! I asked him in the thread, but it is about three years old now. So I don't really expect an answer!

Thanks for all your time! I'll keep investigating...

 

[Fly Dawg]

4 digits out = approx. 36 feet, not too terribly accurate. 5 decimal points out = approx. 3.6 feet.

What you have to remember is that this is not military grade GPS accuracy. The Phantom, even with GLONASS will not achieve that accuracy. Yes, it can be close, but it is inconsistent at best, This also depends on satellite locks. The more you are able to lock on to will increase the accuracy. There are too many variables for this to be consistent at all times. Even 2 or 3 will make a difference. It drives me crazy when people think that if you take off from a point that you will land at that exact same point. Not true. It may, it may be off by 20ft or more.

 

[Lindsaybev]

Even 2 or 3 will make a difference.

I'm sorry, 2 or 3 of what?? Satellites??

So, even if I do find what I now believe to be the Litchi CSV file with 5 decimal points out, and use that information, it is no more accurate than 4 decimals points? How do we know how "accurate" + / - the data actually is?? Is there a place to find a rough estimate of the accuracy? (I wonder how it lands at it's Home Point so well?)

Thanks for your information. I look forward to hearing your response!

 

[Fly Dawg]

I'm sorry, 2 or 3 of what?? Satellites??

Yes, these are calculated in the FC as well. So you have to throw that deviation in as well. There is really no way to be 100% certain of accuracy. As I said this depends on the sat counts and the manner the FC calculates the values. Generally speaking, if your home point is recorded at xxxxx Long and xxxxLat. This depends on the satellite count when it was recorded. For instance if you have only 8 sats when that point is recorded, the RTH value would be calculated at that value. Alternatively if you had 15 sats when that point was recorded the RTH value would be much closer, that is dependent on sat count if RTH initiates. I am still wondering why you are so concerned with the decimal points.

 

[Lindsaybev]

I have a job opportunity with a company which does boundaries, and ownership deliniations for large parcels of land. I just want to be able to have some knowledge of the true capabilities of the drone, and the respective software that goes along with it. They were interested in Pointclouds, but Pix4D is a little expensive for right now. Does the Drone Deploy app have more accurate coordinates or is it based strictly on the drone and the number of satellites?

I feel special talking to a Missile and Telemetry Tech from Boeing. My step-father worked for Lockheed, but he is gone. So these forums really help!

 

[Fly Dawg]

Does the Drone Deploy app have more accurate coordinates or is it based strictly on the drone and the number of satellites?

Only the AC determines positioning. Based on a random flight data log file, which I just looked at, ( You never mentioned which aircraft you were flying ) the GPS Coordinates are 8 decimal places max. That would be the best obtainable IMO. That is what is transmitted via telemetry to the device logs. How other apps interface, interact or display those I am not familiar with.

 

[Lindsaybev]

Only the AC determines positioning. Based on a random flight data log file, which I just looked at, ( You never mentioned which aircraft you were flying ) the GPS Coordinates are 8 decimal places max. That would be the best obtainable IMO. That is what is transmitted via telemetry to the device logs. How other apps interface, interact or display those I am not familiar with.

Wow. That was really kind of you to look! Thanks.

I fly with the Phantom 4 Advanced.

Please don't think of me as obtuse, but one last question:

I find it amazing that you found coordinates that are at 8 decimal places. (I was hoping for 5 or 6.) But didn't you mention that even though it has coordinate values at specific decimals points, that the information is still not very reliable? Or if it records at, say, the 5th decimal point out, can I assume it has a value that is acccurate to the 5th decimal point out?

I appreciate your time, and I find all of this very interesting.

 

[Fly Dawg]

Here is a reference quote: Equations are based on Equitorial. It is not as simple as you think. The AC calculates these values, so another variable. This is all in relation to how far you are from the equator..............

" A value in decimal degrees to a precision of 4 decimal places is precise to 11.132 meters at the equator. A value in decimal degrees to 5 decimal places is precise to 1.1132 meter at the equator. Elevation also introduces a small error. At 6,378 m elevation, the radius and surface distance is increased by 0.001 or 0.1%. Because the earth is not flat, the precision of the longitude part of the coordinates increases the further from the equator you get. The precision of the latitude part does not increase so much, more strictly however, a meridian arc length per 1 second depends on latitude at point concerned. The discrepancy of 1 second meridian arc length between equator and pole is about 0.3 meters because the earth is an oblate spheroid."

 

[Fly Dawg]

On an additional note, if you have not done so previously, you can upload your flight data from your device at the link below for conversion and viewing and see the data for yourself. P4's actually record 3 data files. A .txt and .dat on the device and a much more complex .dat file on the AC itself ( aka a "Black Box" ) all of these are convertible and viewable. Although the link below is only for the device .txt files from DJI GO. Instructions are in the link if you are interested in doing so. I usually download the converted .txt as a .csv and review it in Excel. You will be able to see the 8 decimal places that way. The AC .dat files use a different offline converter.

DJI Flight Log Viewer

 

[bsartist]

Remember. Precision does not imply accuracy. ;-)

 

[Fly Dawg]

Remember. Precision does not imply accuracy. ;-)

Correct..

 

[Lindsaybev]

Got it! Already on it! Didn't get to the .dat file yet. My .txt file gives the number of satellites under the GPS heading, not the coordinates. And I noticed that my picture properties (exif) have very long Lat and Long. coordinates. Wow.

Have you ever put the .dat file onto your video? ( I did that with the .srt. file. I liked having the .srt and .mp4 separate, as this way I could change the font, size, placement. of the separate file before burning it onto the .mp4) I will look for a converter!

Any ideas on Pix4D Mapper versus Drone Deploy??? I guess when one is inquisitive, the questions never end. But have either of you used on of these SAAS programs?

Thanks all! You have helped me immensely!

 

[Lindsaybev]

Here is a reference quote: Equations are based on Equitorial. It is not as simple as you think. The AC calculates these values, so another variable. This is all in relation to how far you are from the equator..............

" A value in decimal degrees to a precision of 4 decimal places is precise to 11.132 meters at the equator. A value in decimal degrees to 5 decimal places is precise to 1.1132 meter at the equator. Elevation also introduces a small error. At 6,378 m elevation, the radius and surface distance is increased by 0.001 or 0.1%. Because the earth is not flat, the precision of the longitude part of the coordinates increases the further from the equator you get. The precision of the latitude part does not increase so much, more strictly however, a meridian arc length per 1 second depends on latitude at point concerned. The discrepancy of 1 second meridian arc length between equator and pole is about 0.3 meters because the earth is an oblate spheroid."

Another referenced quote: ".GPS...a world-wide radio navigation system made up of a constellation of 24 satellites and their ground stations. These 'artificial stars' are used as reference points to calculate a terrestrial position to within an accuracy of a few metres." RIP Galileo!

I read that the first systems using Latitude and Longitude, albeit not nearly as accurately, began in 190 BC! Amazing.

Thanks, Fly Dawg!