[Qgis-user] Did scale change outputting to PDF?

[Andreas Neumann]

Hi again John,

Another thing: what projection (CRS) did you use? Not all CRS are 
suitable for printing to scale - esp. the Web Mercator projections are 
often not suitable.

Greetings,

ANdreas

On 2021-05-27 08:20, Andreas Neumann wrote:

> Hi John,
> 
> A very common error when printing PDF maps (regardless of the software 
> that generated the PDF), is that the print shop scales it down to fit 
> the "printable" area. Then you end up at somewhere around 95% (plus / 
> minus) a bit
> 
> The PDF viewers (at least the Acrobat one) has a setting to print at 
> 100% (but this is not the default) - it has to be done consciously. 
> However, this normally scales uniformly, not separately for width and 
> height.
> 
> On the topic of "base maps":
> 
> Can you explain what source/mechanism for "base maps" you used? Are you 
> refering to "tiled" maps, perhaps consumed through the OpenLayers 
> plugin? This plugin is a known to be a source of error for 
> misalignments and the plugin should be avoided. Even if you didn't use 
> this plugin, I have to say that tiled base maps (e.g. consumed by the 
> quick map services plugin or otherwise) are usually not suited for 
> printing as their resolution is optimized for screen viewing. Printing 
> these tiles would result in disappointing quality.
> 
> Greetings,
> 
> Andreas
> 
> On 2021-05-27 07:15, John Antkowiak wrote:
> 
>> Hi. This plan was too simple to fail - but it failed. The charity 
>> whose project this is needed a large (that is... massive) paper wall 
>> map on which to plot and rethink its delivery driver assignments. Both 
>> drivers and delivery addresses are subject to change from week to week 
>> but it's not a pizza delivery; this is a regular run to supply people 
>> in a bad way. So the plan was to print the base map (roads and road 
>> names and county boundaries only) and then print 8.5 x 11 address maps 
>> with parcel data and orthos. That way, the base maps don't change but 
>> the physical parcel layer is flexible. (On top of that is a third 
>> paper layer indicating which drivers go where so someone can stand 
>> back and take in the whole picture graphically. Not a cutting-edge 
>> state of the digital art solution, but not everyone is cut out for 
>> that. It is what it is.) In order for this to work, the parcel maps 
>> have to be the same scale as the base map. Which they were... in QGIS.
>> 
>> We have to convert all the maps to PDF to print them, and we had to 
>> send the base map PDFs to FedEx/Kinkos to print the 9 map grid panels 
>> at 42" by 62" each.
>> 
>> When we got the big base maps up on the wall, we discovered the scale 
>> did not match the 8.5" x 11" parcel maps output to PDF and printed 
>> from home. It's not off by a lot, but it's enough to be painfully 
>> obvious from a single standard size sheet of paper. I don't know how 
>> to reverse engineer the big map scale precisely enough to enter a new 
>> scale number in the QGIS Print Layout. I didn't foresee it because 
>> this never would've been a conceivable scenario at the engineering 
>> firm where I picked up my meager GIS skills. (ArcMap sent a map 
>> directly to the plotter without interim steps.) There was no scale bar 
>> on the map. It shouldn't have been needed for this.
>> 
>> Did something happen to the map scale when QGIS output the map to PDF? 
>> Could the size of the image on the pdf page have been adjusted 
>> manually or otherwise when being sent to a plotter with 42" paper? 
>> Could the image have been distorted horizontally differently from 
>> vertically? For the life of me, I cannot trial-and-error guess at a 
>> scale to enter. I've gone through dozens of new 8.5" x 11" test maps 
>> trying to guess the correct scale.
>> 
>> Any ideas?
>> 
>> Thank you all -
>> 
>> John A.
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