My secondary goal for georefencing these maps was to provide a web map layer for users to browse historic Cleveland at a high resolution detail (i.e. at zoom level 19-20).
Besides that, I knew very little how I was going to do this and what I could use as the original source map.
The sources of paper maps:
CPL has Sanborn maps. Produced every few years in the early 20th century, these maps richly detail addresses, landuse, streets, rivers, buildings, and often times the owners of these properties, of the entire city. Sometimes the buildings usage was also noted. In addition to their utility, they are relatively asthetically pleasing. They’re also available at an extremely fine scale, a scale of 200 feet per inch.
We also have “Hopkins Maps”, made by a different company but same physical layout and map design.
These maps were published as a bounded book of ‘plats’/’plates’ - pages - each roughly 15 by 10 inches of an arbitrary geographic area.
For the city of Cleveland’s 1881 Hopkins maps, there are 40 pages; each image with borders containing extraneous information (page number, map key) and on several pages, contained areas that are also displayed on another plat.
If I wanted to create a contigous map, I had a fair amount of work ahead of me:
and I didn’t even know what order I should do these steps?!
Edit extraneous areas and clip (or mask) them out:
As shown in the above image, there’s extraneous information (the map scale, the north arrow, the plate numberf) on each page that would need to be removed or clipped out
there are borders (resolution was reduced for file size);
I first planned wanted to stitch them together as one congruious map.
Although the LOC will eventually be uploading the Sanborn map for the entire country, they have barely started the state of Ohio, (save for good ole’ Monroeville). …
So, how do I do this?!
I had multiple questions when I first started:
Do I stitch the plat(e)s together first and then georeference them? Or do I georeference first?
What tools do I use stitch them together?
How much accuracy should I get from them? Is 5 meter accuracy (from a reference layer) realistic? Would a georeferenced plat be
What if the original map had distortions in it in the first place?
Would I be able to get results that are better than this GIF? (after all, I wanted to create a nice digital map layer)
(In this instance OSM is pretty well aligned with State of Ohio aerial imagery licensed in the public domain that is pretty darn accurate [http://ogrip.oit.ohio.gov/ProjectsInitiatives/StatewideImagery.aspx])
I spent an hour or so exploring our scanned maps to determine if there were any that, together would provide enough coverage of the city of Cleveland. Some of their metadata and descriptions our digital collections were misleading; this item has the title of Plat Book of Cuyahoga County, Ohio Complete in One Volume (Hopkins, 1914) but if you carefully read the title page of this book and view a couple adjacent pages of it, you learn that it’s just 1 of 4 volumes that are needed to have complete coverage of Cuyahoga County. Unfortunately, we didn’t even have all 4 volumes of the 1914 Hopkins available; so I couldn’t use that as a resource.
I finally found a map collection that had coverage of the entire city of Cleveland: a Hopkins book of Cleveland from 1881.
So, I started out using the public mapwarper which is really neat.
I experimented by:
Uploading each image page to mapwarper.net (for now, just manually)
Applying the “mask” that would remove the extraneous areas that
I learned that it doesn’t matter whether you georeference or apply the mask first to a map on mapwarper;
This recommendation maybe different if you’re attempting to use the mosaic feature on there.
Lou Klepner reported that Plate Spline is most effective rectifying method on mapwarper; I haven’t noticed definitively one better than the other. For the resampling method, I used cubic spline and didn’t find any noticeable speed delay compared to the nearest neighbor.
I then downloaded the geotiffs from mapwarper - now georeferenced that have the geographic projection stored within them - so they can be displayed over other modern maps.
Now I can open the geotiffs in QGIS as raster layers. they matched up pretty well although not perfect (ADD screenshot) and printed them out. I learned that these image were scanned and uploaded as 72ppi and don’t print well. Our library didn’t save have the original digital scans (they had since corrected this practice several years ago for other scanned maps) You couldn’t read the street names on the printed copy. Oops.
72 PPI is what publicly available on CPL’s contentdm instance but we had 600PPI of these in storage.
Given my limited computing power on my work and personal computers (Thinkpad T450s) each with no more than 16gb of ram); would I be able to work on 1 giant image of all of the items stitched together?
It wouldn’t be realistic to upload about 3gb of images to mapwarper.net…
So, readers: I ask a few questions on how to proceed:
Would would be a suitable resolution to work in, given my two goals (of creating a web map ) ?
In which order should I complete the tasks of cropping/masking the plates, georeferencing the plates, and stitching them together to appear as one image?
What about providing both georeferenced and non-georeferenced images to contentdm ? first and have people want to
Once complete, should we provide both georeferenced and non-georefenced items?
I can see both sides whether to add the georeferenced ones because georeferencing is never perfect; it’s always a work in progress.
Guides that I’ve found by Mauricio Giraldo Arteaga, formerly of NYPL, (http://geo.nls.uk/urbhist/guides_georeferencing.html) https://lincolnmullen.com/projects/spatial-workshop/georectification.html ll assume that you’re only georeferncing one image at a time and not stitching them together.
Later goals: Eventually do cool shit like a historical building inventor
This means, I couldn’t use workflows used when capturing drone imagery which assume the images are continguous manner
(as I was writing this post, I remember mapknitter, so I’ll look into that…)
We have a growing collection of historic maps of Cleveland available online but pitifully aren’t easy to access (i.e. not able to download an entire collection at once, very limited on some items, poor interface) yet.