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Discussion: Contour data

in: dbakker; dbakker > 2017-12-15

Dec 16, 2017 6:03 PM # 
Canadian:
Don't spend too much time yet on the contours. I may be able to nab you some better data. can you send me what you have so I can do a compare?
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Dec 16, 2017 9:10 PM # 
Klepperton:
The existing map has contours - just not sure where they were sourced from, (we also don't know who mapped it).

They seem a little 'off', (at least the contour interval in places), so more than anything wanted some confirmation.

They aren't too profound - a steep river valley with some forested tributaries.

Jeff - where are you getting your data from?
Dec 16, 2017 9:57 PM # 
dbakker:
Jeff - I have sent you the old map my email with that contour data, which is 5m. We are remapping the Campus area to 1:4000 ISSOM, so need a smaller interval. So existing contours aren't all that useful.

I searched around everywhere I could think of (City of Mississauga maps/Open Data, Peel Region Open Data, Googling), but the only data I could get at was from U of Toronto database, where Ian Smith downloaded the City of Mississauga contour data from 2014/2015. He still has it up on his google drive at the link I sent in the email (don't really want to make this completely public). This is the contour data I am currently using, but we (my brother and I) are currently downloading a DEM for the area, and will try custom processing our contours. Never done that before from a DEM (have made it from raw LiDAR though), so we will see how that goes.
Dec 16, 2017 10:02 PM # 
Pink Socks:
Got georeferenced imagery... will supplement with cross checking with 2017 google imagery in the browser (have yet to figure out how to export this from google in a georeferenced format).

Do you need the Google stuff georeferenced? Since you already have georeferenced imagery, can't you just find the same points on the unreferenced stuff and reference them manually? That's what I do.

I also use this tool to get large exports of Google Maps:
http://www.chengfolio.com/google_map_customizer#sa...
Dec 16, 2017 10:23 PM # 
dbakker:
I could referencing the google map based on the older imagery, but I have struggled with this in the past. Specifically for a sprint map I was making in Revelstoke, BC (little town of 5000 people where BC Orienteering Champs will be held this coming year), the newer but lower quality Bing Imagery I was able to get in a georeferenced format, while google imagery was not georeferenced, was higher resolution, but about 12 years out of date. It should have been fine, but the angle from which bing was taken compared with the google map resulted in a lot of distortion across the length of the map so it was unusable. I'm guessing that one of the images was not ortho-rectified properly, but no way to tell which.

Since Toronto is so flat I expect it probably wouldn't be very warped, so shouldn't be a problem - I'll try that. That Google Map Customizer tool looks very useful - thanks!
Dec 16, 2017 10:30 PM # 
Pink Socks:
Yeah, my method isn't perfect, either, as sometimes things don't line up from one end of the map to the other when there's some distortion due to elevation. But in those cases, I just adjust the layer a little bit so that's referenced to the part of the map that I'm working on at the moment.

In several instances, the Bing images I've used have been perfectly ortho-rectified, just not in the highest resolution.
Dec 17, 2017 4:40 PM # 
Canadian:
David, Mark,

The province has a fairly good GIS open data site at https://www.javacoeapp.lrc.gov.on.ca/geonetwork/sr... which has been my go too for much of my mapping data in Ontario recently.

It seems there is Lidar data for much of the province but they've skipped the GTA.

However, if you search for "Toronto DEM" it comes up with some data based on a 2002 orthophotography survey. Not sure if this is the same or different from what Ian was able to provide.

Take a look at that data and see what you can do with it. I tend to open this stuff in QGIS and process it there to get something I can put into OCAD georeferenced to the same coordinate system as all my other data. If you need me to do some of that let me know.

Also, my sisters boyfriend works in GIS in southern Ontario so if you like I can ask and see if he knows anything about Toronto data.

Re photos: There's a tool called SASplanet (http://sasplanet.software.informer.com/14.12/) that allows you to clip out georeferenced photos from a variety of sources such as bing. Theoretically it can also do Google but it always gives me an error when I try to view google photos. I then load that exported file into QGIS and do an export of that QGIS layer with the appropriate coordinate system. I've tried a couple of times to do it without the intermediate QGIS step and there's always been some issue with it. Unfortunately, I don't know enough about georeferencing and coordinate systems and the like to provide much insight beyond saying it doesn't work.
Dec 18, 2017 6:34 AM # 
dbakker:
Jeff,

Thanks! I ended up going with the 2002 Toronto DEM, and then processing contours from that - the data looks pretty good. I don't know what the contours from Ian were based on - likely the same data but the processing was awful.

Tracing is now in full swing - I've got several weeks of that ahead of me.
Dec 18, 2017 2:23 PM # 
Canadian:
Good stuff!

I always avoid using pre-processed data if at all possible. You can always get more what you want if you do the processing yourself. For example, in this case Ian's contours were 1m wheres you really want 0.5m so you can easily get 2.5m contours out of it for the steep terrain.

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