I hope this isn't a dumb question, but I've got a bunch of TIFF images which are scanned railroad valuation maps ("val maps"), some of which are 100+/- years old. I am georeferencing these images onto a GIS basemap, and it's going fairly well.
The thing is that the basemap that I'm using for control points is projected in "web mercator", and I'm thinking I might get better results if I first project the basemap using the same map projection that these "val maps" do. Does anyone know what projection they're using? Or perhaps this varies around the US, or varies over time, etc.
Thanks for whatever thoughts you might have on this.
Web Mercator is dodgy piece of work, designed as a software shortcut - it thinks the world is spherical, not an ellipse.
The official EPSG site ( https://epsg.io/3857) notes
Remarks: Not a recognised geodetic system. Uses spherical development of ellipsoidal coordinates. Relative to WGS 84 / World Mercator (CRS code 3395) gives errors of 0.7 percent in scale and differences in northing of up to 43km in the map (21km on the ground).
It is also regarded as being valid only between 85.06N and 85.06S
Use with caution
I've not known val maps to be projected to anything. You might have to tell us how you got them to show up anywhere near their real world location. I normally scale and rotate them to known ground features I can use to relate to items shown on the maps.
Agree with Jim, Web Mercator is just awful and I would never use it for serious work.
Why not just use a conformal projection designed for the area you're working in?
If not an "official" one like State Plane...just create one yourself.
I've done this before (not with val maps though) and it works very well.
Two words. Rubber sheet. This pretty much illustrates why GIS stands for Get It Surveyed. If you have some real world WGS coordinates along the length of the RR route that the map depicts, you'd have something to warp your map image to, otherwise it is and will continue to remain essentially a cartoon. This is essentially where the survey world and GIS collide and the general public will often confuse the two. Our client uses an ESRI based GIS system based on this World Mercator system which attempts to project property lines and engineers will use these for rough designs and it is nothing but job security for me.
Just because I'm paranoid, doesn't mean they aren't out to get me.
> Our client uses an ESRI based GIS system based on this World Mercator system which attempts to project property lines
Just how far North are you?
Sounds like a receipe for disaster , not job-creation, to me.
I'm at around 62 degrees latitude north. When it comes to placing things on the ground is where we come in. When engineers put too much faith in those GIS lines being gospel is where the potential disasters exists but we typically get involved to survey it before it spins out of control, key word. 'Typically'. Doesn't take many of these disasters to school them on the need to have it surveyed. Hence 'job creation' or as I call it, (relatively) cheap insurance.
Just because I'm paranoid, doesn't mean they aren't out to get me.
I've been making good money from GIS data. Fixing it that is. One of the biggest creators of work for me is Onyx, I get lots of work cause of them.
Hi Jim,
Please note that epsg.io is not the official EPSG registry website, https://epsg.org is. I'm a member of the IOGP subcommittee that maintains it.
I would put all the ones in Tennessee to TN 83 2011 and then send me the link lol. But you can get the ones for Tennessee here: https://comptroller.tn.gov/office-functions/state-assessed-properties/useful-information/railroad-maps.html Somewhere I would like to see this final product though. We are in Tennessee and most of the states that touch it. Sometimes we do work in other states as well. Mostly that is for coal pile quantities. I do a lot of work with easements on our railroad crossings. You used to be able to scrape the milepost numbers from the FRA Safety map. Not sure if you can still do it. Those points are for general location so I would scale maps to them.
I can't speak for what the Railroad data is in, but I do want to speak up regarding the association of Web Mercator to GIS professionals. Many of us are keenly aware of this PCS that enters into maps when the default ESRI basemaps are used. The first thing to do is to control the Map spatial reference with a standard (shall I say survey grade) PCS like a lambert conformal projection. I'm here in Alaska, and a decent one for statewide work is Alaska Albers NAD83 (2011) and of course SPZ for fine spatial scales. This notion of Web Mercator was for invented speed of draw and tiling focused on the online (ArcGIS online) crowd and is in use with many vendors (Trimble Maps for one). Not saying I'm speaking for every GIS person out there, but plenty of evidence to its use is out there in many of the forums to the contrary. Kudos to Melita Kennedy and her projection team in building an incredibly robust GCS and PCS library which is at our beck and call to project (if needed) or define projections (the ideal first case) in mixing data (a daily job for GIS). Proper GIS software use can make surveyors money, and its my fervent hope many in our industry are using this excellent forum to ground ourselves with you - the professional surveyor.
I concur with rubber sheeting the tiff or jpgs of the rr map into some real world objects you can match to a nicely referenced aerial. Might be a street crossing or the start of a spur line. I referenced images of all the rr maps in my county into autocad using rubber sheeting. And only did that to provide some approximate special presence to the maps. So I didn’t have to look through hundreds of unorganized rr map files to find the rr map near my project.
If I need to get into the rr map further then I try to reference it with something with more of a survey grade coordinate.