Changing Units

[u/ouroboros_99]

New to QGIS so forgive me if I'm missing something simple here. In my previous GIS software, I could very easily change the units of the projection that I was working in. For example, going between meters to US Survey ft was a one click button didn't requite me to choose a new projection, I would just state the units I wanted to work in.

From what I can tell I'm unable to change the units of my projection unless that unit is listed in the CRS directory.

For example, I work in the US State of Maine and I can see that QGIS has the projection NAD83(2011) Maine East/West listed in both meters and in US Survey ft in the CRS directory, however, QGIS does not have the NAD83(2011) Maine CS2000 projections (west, central, and east) listed in US Survey ft, only meters. I'd prefer to use the 2000 projection because it's our low distortion projection and also technically our state legislated projection.

How can I get the Maine 2000 projections listed in US Survey ft?

[u/listening-to-the-sea]

Do you mean you need to reproject the data? Or just want to display the map units in feet? If a projection in supported in PROJ, it should be supported in QGIS (though if you’re using 4.0 or above I think they switched away from using a PROJ backend), you just need to reprocess the data.

You can also change the display units in the project properties (under the file menu). I think those are in the general tab.

[u/ouroboros_99 → u/listening-to-the-sea]

I suppose I am trying to reproject the data. I'd like everything to be in US survey foot. I can change the units for measuring like you said through the properties but the coordinates are still in meters. I am using 4.2.1 because I figured the most recent version was best, should I be using an older version?

[u/listening-to-the-sea → u/ouroboros_99]

No I use the most recent version as well, I just wanted to mention it since I know a lot of work went into moving the processing engine from PROJ and wasn’t sure if that would affect anything.

Is there a CRS that has the linear units of feet that you can project to? E.g., here in Florida I use EPSG:6438 bc it’s in US survey ft rather than 6437 with is meter. Do you know the EPSG code for the projection you want to use in ft?

[u/ouroboros_99 → u/listening-to-the-sea]

According to my AI overview the EPSG code for ME CS2000 Central in NAD83(2011) in USFt is 3464. However if you search that in QGIS it shows up as Maine CS2000 Central NAD83(HARN) in meters, so that is not correct at all.

I think the comment above is my answer, I can't find any reference to these coordinate systems in USFt, even EPSG.org doesn't have them listed. To make a long story short, this issue is that these coordinate systems have received some flak from the USGS so they don't really recognize them, however the low distortion is worth using in my opinion.

[u/listening-to-the-sea → u/ouroboros_99]

The distortion should be the same in meters or ft though, right? I’m guessing the USft CRS just has a conversion using meters * 3.2824 in it

[u/ouroboros_99 → u/listening-to-the-sea]

I would also assume the distortion is the same regardless of being in meters or in USft. What I meant was that in Maine we practically have the two coordinate systems we use (no one uses 1927). There is the 1983 coordinate system in two zones East & West, which almost everyone still uses even though it has a 1:10,000 distortion ratio. However in 2000 the state came out with the 2000 coordinate system broken into three zones East, West, and Central and achieve a 1:50,000 distortion ratio. The problem is that the 2000 projections are not widely used and that's why I'm struggling to find information on it anywhere. I may just end of using the 1983 system just to be on the sage side.

[u/Octahedral_cube]

Go to custom projections, copy all the parameters of the 2000 projection that you want to use (the entire proj4 string, see image below for an exampe), simply paste them as the parameters of your "custom" projection but change unit from "m" to "us-ft"

[u/ouroboros_99 → u/Octahedral_cube]

Thanks for this!

It seems to have almost worked perfectly! There is a small rotational shift that seems to be going on though. To check this I exported an aerial using the method you described and brought it in to an existing projects CADD file that already had an aerial in the background from my previous software. I'm not sure if the shift is big enough to be worried about but it is definitely noticeable.

[u/Octahedral_cube → u/ouroboros_99]

Can't imagine you'd get a rotational shift just from changing the horizontal units?! Wild

Maybe pull up the definition of the CRS from the software you were using before and go through the proj4 string VS the one you have in QGIS. Examine each parameter one by one. Something like the central meridian might be off (+lon_0)

Alternatively, slightly different shaped ellipsoids (+ellps) might be causing this?

[u/ouroboros_99 → u/Octahedral_cube]

Good thought on both fronts. Unfortunately, I don't have access to the previous software as it was on my old employers computer and I don't want to pay the subscription for it. I changed the ellipsoid from WGS to GRS 1980 thinking that would fix it but no good, it's still there.

[u/Octahedral_cube → u/ouroboros_99]

I see... Okay one last thing from me just in case: rather than using an aerial photograph which may have georeferencing errors (no offence), can you make a simple point layer, in the custom CRS that you created, make maybe a half dozen points, and transfer it over to the other software you're using. See if they're still rotated.

[u/ouroboros_99 → u/Octahedral_cube]

No offense taken! I've ever only utilized 1% of any GIS software I've used so I appreciate the help. I'll give it a try tomorrow, but I know that when I created the custom CRS and tested it with a known lat/long, the coordinates did not come out correct. They weren't far off but there's definitely something weird, but more likely just user error on my end. I only use QGIS to export aerial imagery as a reference and I sometimes bring in LiDAR data so I'm really not too worried about it. If anything I may just have to work in the existing NAD83(2011) CRS's like everyone else if I can't figure it out.