LDP/Custom coordinate systems

Eric Tweet

Active Member
Howdy, Survey Crew!

Just got a job which will be using a LDP, so I started poking around in the LS+ to see if there was a straightforward way to simply drop the parameters in and hit the ground running.

It appears that there are only "common" grid systems that are able to be used (State Plane, UTM, etc), but no way to adopt an actual LDP by selecting a generic projection, and applying the parameters of the system I was given. For context I'm trying to create a system using the Lambert Conformal Conic (1 parallel), and the usual parameters like False easting/northing, and so forth.

Of course even if there is no way to achieve an LDP using that approach, I have no problem going the localization route (or State Plane in the field and convert in the office), so this isn't really a critical concern. Just wanted to confirm this is not a feature that's available. Much like the last slice of butter, I'm spread pretty thin these days so it's entirely possible I've overlooked something :)

Nate The Surveyor

Well-Known Member
I don't know that this will answer your question, HOWEVER....
I have regrets. My regrets are that I did not START out doing this for all jobs, in my area.

Here is what you want to do. Or something like this. Start a job, call it SF study. (scale factor study). Now, drive all over your area, and store coords. Name these by the town, or area of that coord.
I can see names like: Mt Ida. Glenwood, Hot Springs, etc.
Then, look at the scale factors for all these points. Average them. Or, make a couple of groups, if there is too much difference.

Now, a new job comes in, and you can assign that LDP to that job. You can even put a single rotation to them all, so that their orientation is other than GRID north.
An accutely simple way might be to select a scale factor, (CSF) for your house, and impose it on all your jobs. This will work up to a certain distance from your house. (depending on type of SPC it's based on, Lambert or whatever, and direction, N-S, or E-W

Now, make a single job, with these 3 LDP's, and USE this job as your Template, that you can now use any time. And, select the page, with the needed LDP, and work in it.
Maybe somebody more experienced can suggest ideas to make this fast and workable.

I have the ability to MAKE a LDP. It looks like this:
LAT LON>Arkansas State Plane South Zone>LDP, (These are custom parameters) Just like what you see in localize.
You can COPY this same LDP to many Jobs. Now, you have your own custom LDP. Or, 3 of these, (up to 10) and this will allow you to do ANY job, and ASSIGN it to a chose LDP, based on location etc.
I don't have this going on, but IF I'd learned this early on, it would be very useful now.

This method would allow my to SELECT perhaps a local system, for that area. And, this would make it simpler, by copying the SAME LDP for dozens of jobs.

Just a thought.


Eric Tweet

Active Member
Excellent thought process! And in fact that's exactly what I use, in one form or another, for almost everything I do. Sometimes it's just a "single-use" localization, or other times I use a localization which covers the entire municipality (it's a small one, don't worry!). Actually the town has it's own "official" coordinate system which is tied to physical monumentation, so that makes a variety of tasks easier when localizing to that system is required to get the job done.

An interesting topic arises when differentiating between the 'Localization' type of LDP/coordinate system and something more along the lines of a "publishable" conventional projection type of system. On one hand we have the Localization where the system is defined by a best-fit of our measurements to assumed or design coordinates, with a basic underlying grid projection being used to make that fit happen. And on the other hand we have an explicitly defined projection based on a selected projection (Lambert, Mercator, Stereographic, etc), and a set of parameters (False easting/northing, lat/long of origin, meridians/parallels, scale factor, etc.). Both of those methods can benefit from field measurements in order to facilitate determining parameters or points (depending on the aforementioned type of "LDP" we want) which yield a system with desirable characteristics, be it low distortion in length/angle/area (LDP!), or some other feature.

Indeed, as you point out, Nate, measuring points is key to investigating just what numbers are needed to punch into J-Field to get a good system in place for a given task. I've got a handful of localization files saved up for jobs in different places. Some of them are fairly strange, with atypical definitions of North, and so forth (engineers are an interesting bunch, aren't they?) :) Whatever the job requires!

All of that being said, the question remains: when a client provides you with a parametrically-defined LDP, can J-Field wrangle that? Or is it necessary to only use the Localization approach in order to get the LS+ surveying "in their coordinate system". For example, how would you instruct your LS+ to deal with a client who says "please use this LDP: "

Projection: Lambert Conformal Conic
Latitude of Origin: xx-xx-xx
Longitude of Origin: xx-xx-xx
Central Meridian...

All stuff I'm quite familiar with handling in various GIS/CAD programs in the office, but in the field with my LS+ is what I hope to nail down...

Shawn Billings - Javad

Active Member
You can absolutely create a custom projection in J-Field. I do it all the time.

Go to the Coordinate System button on the Second Home Screen


Click the "+" button to add a new coordinate system


Select the Geographic System for you projection


Using Near Me will reduce the selections to those systems that are near your current position (provided the LS can compute a position from available satellite signals)


Note that geographic systems have a globe symbol


Select an appropriate Geoid


Select the "Custom Projection & Rename" to enter the projection type and projection parameters.


Select the button below the name to toggle through the available projection types, enter the projection parameters and enter a name. Press OK and Done.


You can make the custom projection one of your "favorites" for easy recall by highlighting the custom coordinate system in coordinate systems and selecting the resource button (three green bars) and selecting "favorite". Then you can recall it for future projects.

Eric Tweet

Active Member
Fantastic, Thank you Shawn!

I knew it had to be right under my nose, and indeed it was :)

I imagine one of my hurdles was that I was expecting to select things in the reverse order: Projection first and then applying a geodetic datum to that. One more thing to add to my "ahh, now I know!" list.


Active Member
Your the man Shawn! Thats what I had been tinkering around with when we talked about the it at the seminar.

For what its worth, it should be pretty easy to create a bunch of random points in GIS to get an average scale factor. As Shawn pointed out, if you have a very prominent river valley or high mountains, you would need to give those areas a little less weight in the average as to not skew the numbers.

Shawn Billings - Javad

Active Member
One note I'd like to add regarding Low Distortion Projections. I created this projection quite a few years ago (well before the Triumph-LS was released). In the intervening years I came to the conclusion that projection scale factors beyond the sixth decimal place are perfunctory (I never get to use that word, so there it is). The sixth decimal place is at the single digits of parts-per-million (ppm). An elevation change of 21 feet creates 1ppm in scale factor change. You can imagine how the places beyond the sixth decimal place are beyond practicality. So if I were designing that projection brand new today, I would make that scale factor 1.000014. But because I had already created this projection with a scale factor of 1.00001378, I continue to use that value to match new grid coordinates to previous grid coordinates. I can't now round the value to six places. Once it's established, it's established, but future projections that I create will be with scale factors at 1ppm.

Shawn Billings - Javad

Active Member
As Shawn pointed out, if you have a very prominent river valley or high mountains, you would need to give those areas a little less weight in the average as to not skew the numbers.
That's a good reminder. Sometimes you might not want to strictly average the elevation for a projection area. You might weight the average elevation toward the population or where the majority of future work might be. There are few strict rules about designing an LDP but there are some good guidelines to consider.