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Discussion: Satellite image with World File

in: Orienteering; General

Jul 2, 2016 1:54 AM # 
I want to set a few courses that can be printed either on an orienteering map or on an air photo. In Condes. To do this properly I need an air photo that has a World File attached to it. How do I get that? Cheaply.

(for those that don't know, this is what Wikipedia has to say... A world file is a six line plain text sidecar file used by geographic information systems (GIS) to georeference raster map images. The file specification was introduced by Esri, and consists of six coefficients of an affine transformation that describes the location, scale and rotation of a raster on a map.)
Jul 2, 2016 2:36 AM # 
I don't recall Condes, but if it's an orthophoto, could it be as simple as bringing the photo into Ocad or OOM as a template, and adjusting the template to the map? Then change the opacity of the map and template. (I don't recall whether Condes respects those. )
Jul 2, 2016 3:20 AM # 
Can you download aerials from a government website? Or get them from a local government GIS office?

You could use a GeoTiff (extension tif or tiff) with no world file, but you'll need to know the projection. The world file info is often embedded in the tiff, so the data is there, it's just not a separate file.

Typically when you get aerials that aren't GeoTiff, it will come with the world file, with extension wld, jgw, pgw, etc., where jgw is for jpegs, pgw is for pngs. Again, you'll need to know the projection. Sometimes, there is only an xml file that defines the world data and projection and other things. I think QGIS uses it, but I don't know about OCAD. (Or are you using Open Orienteering Mapper?)

If your software is competent, it will handle the projection of the image. But if it isn't, typical of older OCAD versions, you'll probably have to use something else to reproject it to meters and a projection matching your other data. I use QGIS, which is open-source and free.

Another option OCIN uses is Pictometry. There was a push from OUSA for clubs to get accounts for a nominal cost. It's like 18 dollars an hour of website time, which is kinda stupid, but it works. Those tend to be small images and you have to download a lot of them and then use QGIS or something to create a mosaic image. I'm assuming they also have aerials of Canada, but I'm not certain.
Jul 2, 2016 4:37 AM # 
Terje Mathisen:
I tend to use Google Maps in sat image mode to download a bunch of images, then merge them in GIMP/Photoshop before loading the image as a background in an OCAD project.

After doing this I can re-export (a part of) the image with world file from OCAD by hiding the map in draft mode and only show the background image.


PS. For the first step it really helps to have access to a 4K monitor since that reduces the number of screen captures needed by a factor of at least 4 compared to a regular "Full HD" screen.
Jul 2, 2016 5:38 AM # 
I also do what Terje describes... it's a bit of cheat but it allows you easily get it to match the map file using the adjust background map tools (F9) in OCAD.
Jul 2, 2016 3:42 PM # 
What is the location of the area you are interested in?
Jul 2, 2016 3:57 PM # 
The specific area this time is Pearce Estate in Calgary. But I'm trying to understand the geo-referencing world better and I'm looking for a general purpose solution (for Western Canada at least).

I see that GlobalMapper software has this ability ("Capture Screen Contents to Image" command), but it costs around $500.
Jul 2, 2016 4:51 PM # 
Something like this?
Jul 2, 2016 5:07 PM # 
I'd talk to the local government there. With the floods, I'd bet money they've had top-notch lidar and aerials flown recently. (Some flood plain lidar is ridiculous. You have to throw away data just to process it at a reasonable tile size.)
Jul 3, 2016 12:57 AM # 
AZ, what's your present knowledge and use of geo-referencing? Where does your mapping base material come from?
Jul 3, 2016 4:33 AM # 
AZ, using the link I posted above, I could find no aerials for that part of Calgary between 1900 and today. I did a similar search on Whister Mountain, and the most recent result was 1993. It requires registration to "add to cart", and I'm not too enthusiastic about trying it.

Here in the US, I'm used to free data, with some exceptions. I'm not sure if this sort of stuff is ever free in Canada.

Again, I'm going to recommend calling or walking into the local government GIS office. Here in the US, I can say I'm with a non-profit and tell them it's for a local park and usually get free data, or a reasonably-priced map. Although, local gov't offices are used to dealing with construction and architectural people, so they're used to people paying for maps and data, and we've had to pay in the past.

I found a 2007 aerial on Pictometry. I'm seeing 2016 copyrights on several map websites. There are significant difference in the park between 2007 and 2016. The biggest is the loop road south of the Hatchery building. It's bigger now.

rant: What is up with the aerials on Google Maps? The 3D feature gives those horrible compression artifacts. The 3D viewing itself is pretty cool. I just wish they'd turn it off in ortho mode and just give me the best damn aerial they've got. /rant
Jul 3, 2016 4:56 AM # 
@gruver - great question. My present knowledge is very, very low. And this is my first use of geo-referencing.

I'm using orienteering maps made by Remo Madella - and he collected the mapping base material, I believe using SAS Planet and Global Mapper.

(Remo gave a presentation about the gps-based mapping he did, and much of it was focused on geo-referencing the various materials - I'm trying to actually do what he talked about but wondering if I can avoid the software costs).
Jul 3, 2016 4:59 AM # 
@cedarcreek - thanks so much for this link, and the emails you sent me. Like you I couldn't find any data via the link - how not useful is that ;-( And the files you sent me are great, but like you say the changes due to the flood are significant and so I can't effectively use the pre-flood images.

There must be data somewhere - I will check with some friends who have friends in Calgary Parks department...
Jul 3, 2016 5:57 AM # 
I normally would not recommend using the F9 Template Align function, but with that georeferenced 2007 image as "true", you *could* screen shot Google or Bing or whatever, and align it to the "reliable features" in the 2007 aerial.

Honestly, though, the local, state/provincial, and national government is where you want to go. Often the online maps are very compressed. Often they're the summer "agricultural" aerials with reduced resolution. The leaf-off aerials tend to be higher resolution, and that costs more, so they don't fly them as often. It's common in the US for the agricultural series to be flown every summer. The good stuff is worth getting. I've had mappers tell me they're shocked at how good it is compared to what they usually get.
Jul 3, 2016 12:36 PM # 
Terje Mathisen:
I've had success several times using OpenStreetmap data as my geo reference, adjusting images to fit that baseline.

As long as the resulting O map is open access / not-for-profit and you send some improved data back to the OSM project I believe they will be OK with this use.

Jul 3, 2016 12:42 PM # 
AZ your needs might be of two sorts - knowledge about what geo-referencing is about (in the orienteering context), and sources of suitable photos. I can't help with the latter but offer some advice for the former. Sorry if this is "old hat" to you. I also apologise to the gurus out there for my simple view:-)) DON'T chime in with all the fancy stuff until AZ confirms he's ready for it.

Our mapping software is pretty much built for the DRAWING task. Magnetic north defines the coordinate directions, and everything is stored in mm on the map. That's not the way the real world works. It uses true north, and metres on the ground. Stuff from a non-orienteering agency will be in these "real-world" coordinates. This might be photos, contours, spot heights, tracks and waypoints from your GPS.

OCAD and OOM provide a link between the two. Where (in a country's coordinate system) is the middle of the map? And what is the angle between magnetic north and the country's grid north? (Let's pretend we're in a country small enough to have a single rectangular grid.) In OCAD you can define these numbers in the "set scale and coord system" in the map menu.

If this has been done (and Remo undoubtedly did it for you) then you can switch your OCAD file into "real-world" mode and see what the coordinates of any feature are. You can put a point feature at given coordinates (clunky but it works). You can open a digital orthophoto* as a background image, and the tiny 6-line accompanying world file will tell OCAD how to position it. Then (I presume) you will hide the map, or some of the map, and carry on into Condes.

* Orthophoto - corrected for all the forms of distortion, and made to match the country's rectangular grid.

Before anything else, does this make sense? And just one more question, did Remo use any aerial photos in making the map? I don't want to watch 2 hours of video to find out heh heh.
Jul 3, 2016 10:47 PM # 
I was asking about location because what is available for municiaplities in western Canada varies greatly. Edmonton and a large number of BC cities in the lower mainland as well as Prince George and Kamloops offer map data, orthos and/or Lidar for free with no resricitons. Calgary is not among those unfortuately. I've also found some, though not all, GIS departments in some smaller municipalities are quite happy to provide what they can even if it's not available online.

I recently purchased orthos in Alberta from Valtus. Altalis is a reseller in Alberta Subject to tthe minimum of $87 its pretty reasonable as you can crop your purchase to your AOI and only pay for what you need. I think I paid $160 or so for 15 sq km of recent high resolution colour orthos. Seems to me you may you may be able to aquire more than one block in a single purchase and be charged by total area.

This discussion thread is closed.