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Long-awaited FizzyCalc update


fizzymagic
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FizzyCalc 1.2.01 now does datum conversions. Download the new version and give it a whirl.

As I stated in another thread, thanks for the update! However, my first session using the datum conversion has me wondering a bit about what is going wrong. I suspect that the problem lies with the maps we have rather than the conversion software.

 

I picked a cache in the vicinity of Invermere, BC for my first conversion target. This is one of the towns used as a sample in what we have been using to rough in conversions from WGS84 to NAD27 maps. When I placed the coordinates into the converter and selected the datums (WGS84 to NAD27) I quickly received the corrected coordinates to plot. This measured out to a point about 80m to the east and a few metres to the north. The conversion chart for BC at that location shows about 80m to the east and 200 m to the north.

 

I have been prepping a few other samples to try, so I'm not going into details much right now. But I suspect that something is amiss somewhere in the process. The E conversion seems to be about right, but there is a difference in the N portion. Also the web says that the amount your system and others corrects for N is about right for USGS and other US maps. Which leaves either, Canadian NAD27 maps were further out of correct measure (which is likely given the remote areas here), or there is some glitch in someones math for the conversion or chart. Certainly a shift of one decimal place on the N would take 200m easily to 20m, but which is which in that case. I'm interested in this since the whole thing for me was to have a handy conversion tool for SAR purposes, since we have lots of NAD27 vintage maps around the province still, (there is a new printing pending but not quite yet).

 

One can see the problems we have if there is a glitch, so I want to find out what is correct for us to use. And to make people aware of what it is and why it should or should not be used in OUR circumstance. I will say that our chart gives only two choices of NAD27 to chose from and both are within 1 m of each other. Some have found the Alaska variant works better in some areas. So that might account for some of it. But I suspect that would be more for MSL measurements (for conversion to Marine Chart for depths and heights above water at various times of day) rather than for land navigation.

 

I'd find it hard to believe that there is a great difference between land versions of NAD27 even in Canada, but I can believe that some of our survey work was out a fair bit, since it was done by aerial photogrammetry mostly with some survey re peaks etc.

 

On the other hand, slippery decimal places are common in math as well aren't they? For me they wander more than one place at a time on occasion.

 

Time will tell, I hope.

 

Doug 7rxc

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As I stated in another thread, thanks for the update! However, my first session using the datum conversion has me wondering a bit about what is going wrong. I suspect that the problem lies with the maps we have rather than the conversion software.

 

I picked a cache in the vicinity of Invermere, BC for my first conversion target. This is one of the towns used as a sample in what we have been using to rough in conversions from WGS84 to NAD27 maps. When I placed the coordinates into the converter and selected the datums (WGS84 to NAD27) I quickly received the corrected coordinates to plot. This measured out to a point about 80m to the east and a few metres to the north. The conversion chart for BC at that location shows about 80m to the east and 200 m to the north.

 

I have been prepping a few other samples to try, so I'm not going into details much right now. But I suspect that something is amiss somewhere in the process. The E conversion seems to be about right, but there is a difference in the N portion. Also the web says that the amount your system and others corrects for N is about right for USGS and other US maps. Which leaves either, Canadian NAD27 maps were further out of correct measure (which is likely given the remote areas here), or there is some glitch in someones math for the conversion or chart. Certainly a shift of one decimal place on the N would take 200m easily to 20m, but which is which in that case. I'm interested in this since the whole thing for me was to have a handy conversion tool for SAR purposes, since we have lots of NAD27 vintage maps around the province still, (there is a new printing pending but not quite yet).

 

First of all, there is an explicit Canadian NAD27 datum in the list. Did you use that one or the US NAD27 mean (the one that is just labeled "NAD27?"

 

But the symptoms you describe (correct longitude shift but incorrect latitude shift) is exactly what you see when the Molodensky parameters are correct but the ellipsoid is not. The US NAD27 ellipsoid was based on the Clarke ellipsoid of 1866; it is possible that the maps you are using were based on some other ellipsoid.

 

Nonetheless, if you find an error I will be most happy to correct it!

 

-- Dave

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First of all, there is an explicit Canadian NAD27 datum in the list. Did you use that one or the US NAD27 mean (the one that is just labeled "NAD27?"

 

But the symptoms you describe (correct longitude shift but incorrect latitude shift) is exactly what you see when the Molodensky parameters are correct but the ellipsoid is not. The US NAD27 ellipsoid was based on the Clarke ellipsoid of 1866; it is possible that the maps you are using were based on some other ellipsoid.

 

Nonetheless, if you find an error I will be most happy to correct it!

 

-- Dave

Okay on that. I had used the first NAD27 I saw. Will see what the Alaska/Canada version does.

The ellipsoid thing is quite possible. Like I said earlier I'm more than happy to assume the maps are simply made wrong in the first place, but more than good enough for when the datum was created. I will see if I can find out about the ellipsoid version.

I was reading something in that line from NRC Canada but don't remember that name mentioned. I have edited down a bit of the BC conversion chart (which I stress is for adjusting one set coordinates for one series of maps). No way was it meant to reflect badly on a good product, which even I can see matches other sources for the same conversion. I'm just trying to see if WE need a new conversion chart... or better yet an up to date series based on at least NAD83. This is a very wild area and much on the map is not what is on the ground these days. I heard recently that the new maps should be on the way, maybe this fall.

 

Meanwhile I will try to do some experimentation to see what is what. Thanks for the update information. And the word is out now that not all datum conversions will do the job when something is wonky somewhere. I'll have to see if there is a new CC as well.

I intend to post my cropped form here once I post it to my online storage.

 

Doug 7rxc

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Lots to read up on web wise! Let's see... well the NAD27 versions of the National Topographic Series maps were supposed to be made using the Clarke 1866 datum. However, reading a few sites I finally came up with one that lists datums by name and includes some(all?) of the things that define them. I didn't know there were that many. Still comparing them and while I have only a vague idea how they are fit into things mathematically so far, I can see that most are fairly similar. However I did find one that seems to be for the area in question and has a few conversion numbers that fit what I suppose would fit what my paper chart gives, maybe.

I'm now looking for information on how they apply the numbers since what I imagine and reality might be different. The one I found is for 'NAD27 Alberta and British Columbia'. Didn't see that one on the software list so I can't test it there. I am sure that the ones for the places like Mexico and southern States won't do it. Time will tell I guess. Also on the same site found information on various ellipsoids some of which vary in amounts that might do the N/S shift if they were applied the way I imagine. Need more information on that part as well.

 

Only constructive thought so far. I originally missed the alternate NAD27 listings, due to the fact that I'm used to having the NAD27 part as a prefix (I use GARMIN GPS which does that). Putting the country/area first is also logical, but not what I was looking at.

So I'd be a fan of listing the NAD's like this... NAD27 Conus or NAD27 Mexico, but that is just me. NOW that I know how they are listed in the datum conversion section it's not a problem beyond searching the list by area.

 

One other thing I noticed is if you enter a UTM coordinate set into the first box it will give you a good result, but does it in DD MM.mmm format. Not a big problem to reconvert in the conversions, but either a list of input/output choices OR simply matching the format of the input might be a future consideration. Meanwhile it's just cut and paste into coordinate conversions.

 

Probably the whole NAD27 conversion thing will fade away except for people with old maps as the newer ones appear anyway. At least I'm getting 'educated' on some things I never considered before.

 

Doug 7rxc

Edited by 7rxc
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Well, after lots of reading and some limited testing in the field with more to come, I can say that the problems definitely seem to be with the older datum maps.

 

FizzyCalc and other converters prove to be both fairly consistent and work well for newer maps. I'm quite happy with those results.

I would like to see a good match between available Datums on the lists, my current GPS offers different appearing ones to the software listing. They have specific names and those should be used for clarity. This might be the fault of the GPS makers, the Datum makers or whoever. It is just a bit confusing since you use apples and oranges at times, or just twist them around.

 

Our 'supplied' paper chart seems to fit OUR topo maps better. I finally got access to a really local map and did some comparisons for easily identified landmarks. Two within several kilometres of each other. Both plot close to the chart which does not have our location as an option (closest is about 100 km to the west). However, they vary in the offset considerably. A shows about 10m E and 310m to the South when the NAD27 coordinates (in UTM) plot on GE, the other (which is slightly NE a few kilometres) B shows 98m E and 265m to the South. I suspect that the chart was done on that basis by local SAR groups then grouped into a Chart of conversions.

Interestingly, while reading about various datums etc, I found that these values appear close to the X and Y offsets provided in the descripions but don't seem to be processed by any of the formulas, just applied directly. Still that seems to work for OUR maps.

 

As I said before, there is a newer printing, hopefully from new data (the newest of the other series was from 1970's) and are pretty out of date on the ground. We'll see someday. Meanwhile, I plan to keep doing test points between NAD27 maps and WGS84 and will make up my own local conversion chart and see how that works out.

 

I also have to figure out what is happening with the GPS. Mark a waypoint in WGS84 of course, then change the setting to one of the Datums that appear to be the same (Garmin/Fizzy lists) but the results also differ for me. While the numbers should not change, the GPS should take you to the other location as GZ as I understand it, but while that happens to a small amount, when I use the coords in FC I get different results even with the apparent same Datums and direction of conversion. I'm wondering if Garmin also has things messed up or is just using some simplified conversion as well. And yes I know that GE has problems with fitting maps/photos to the coordinates in some areas. Again,I suspect that theory is one thing and practice is another. Here I had difference of direction and distance as well. Will keep playing around though, since most of my problem was NAD27 MAP conversion.

Perhaps GE doesn't use WGS84, I'll have to check.

 

Thanks again

 

Doug 7rxc

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