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Discussion: Newbie ISSPrOM map making quetions

in: Orienteering; Gear & Toys

Aug 1, 2020 8:01 PM # 
barb:
In using OCAD's Wizard with OSM data, do you generally accept the rotation (magnetic declination) or do you look it up?
OCAD gave me like 12.9 and NOAA gave me 14.34
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Aug 1, 2020 8:15 PM # 
barb:
Also, I'm trying to use the New Map Wizard and it doesn't bring in all of the data that I see in the selection window. It cuts it off at the top and bottom. ... OK, I just zoomed out and made the area way bigger and then I brought in enough. What you see in the selection doesn't always match.
Aug 1, 2020 10:05 PM # 
andreais:
- use much larger area (the cut-off has happened to me several times in eastern states) and then draw a shape of what you want to keep with an area symbol, select shape, Map->Export Part of Map, and then work with that new partial map
- use what ocad gives you because that already takes into account that the rotation is the sum of the grid convergence and the declination. The declination for Cambridge, MA is -14.34 (W), the grid conversion for 0 declination is +1.43, giving you rounded -12.9 in OCAD for the grivation or magnetic rotation
Aug 1, 2020 11:13 PM # 
barb:
thank you!
Aug 2, 2020 1:35 AM # 
gordhun:
That certainly is the fix but it is a band aid solution, not what we pay OCAD for. They are regularly updating their program with fixes of reported problems. Have you reported this cut-off problem to OCAD?
Aug 2, 2020 10:03 AM # 
robplow:
https://ocad.com.au/grid-magnetic-angle/

That was written before OCAD introduced its magnetic declination function into its wizard.
Aug 2, 2020 5:03 PM # 
cedarcreek:
I use the WWMGUI software free from NOAA.

https://www.ngdc.noaa.gov/geomag/WMM/soft.shtml

If you input coordinates in UTM (I use the website Acme Mapper with options set to show coordinates in "UTM"), you can read the grid-to-magnetic "grivation" value directly. What Andrea says is correct, but I never do the math---WMMGUI just gives me the number. But it appears OCAD is also giving you the right number. "Declination" is to true north, and if you use UTM coordinates (which are conformal and convenient), grivation matches declination only at the center of the UTM 6 degree "orange slice".
Aug 2, 2020 5:23 PM # 
andreais:
@cedarcreek - I have found magnetic declinations changing from 0.03 to as much as 0.1 per year; maybe more? Are there recommendations anywhere clubs are following on how often to adjust maps?
Aug 2, 2020 5:33 PM # 
cedarcreek:
I haven't seen any. I've thought about it. I can't imagine it matters until the map is over 20 or 30 years old. At your 0.1 degrees per year, that's 2-3 degrees.

I recently rotated a map that was made in 1984 (I think) but later redrafted without changing the north lines. It was a pain, but not terrible. We needed it to be georeferenced for use in UsynligO. I separated the map from the border and legend elements into separate files, rotated and georeferenced just the map part, then imported the legend and border elements and readjusted the items that were in confict. The biggest problem was that the old map was from aerial and USGS contours, and it had significant distortions. Even the new aerials weren't the best, so I ran lidar and used that to line up the old map details with the lidar details. It took several tries because one of the map items was a fountain, and it was displaced by the mapper a bit for legibility in the surrounding sidewalks. But the big errors were in the woods away from stuff. It was good enough for UsynligO though.
Aug 2, 2020 7:55 PM # 
mikeminium:
In parts of the Midwest, we were seeing a pretty fast shift in declination, in the range of 5 degrees in the last 40 years or less. I routinely show my school classes 2 slides of 1994 and 2004 declination, showing the zero degree line shifting to the west by well over 200 kilometers, from the Indiana-Illinois border to the Illinois-Missouri border near St Louis, in just 10 years. A couple decades before that, the zero line was another 250 km farther east near Cincinnati.
Aug 3, 2020 12:18 PM # 
robplow:
2-3 degrees is certainly enough to effect your navigation - even it is not easily noticeable to the runner. I would considered anything over 1 deg error too much.

When I was mapping in Whitehorse I helped someone get a map that hadn't been used for some years ready for an event - the mag north was out by around 6 deg if I recall correctly. The annual change there is currently 20min (ie 1 deg every 3 years).

Certainly if you are somewhere where the annual change is high you should have a policy on keeping maps up to date regarding declination. It is pretty simple to rotate the map, You have to think about the north lines and layout - make sure they don't rotate with the rest of the map. And the bigger the rotation you might find you need to redo the layout slightly.

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