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OCAD Adds Co-ordinate System for Nearmap

NearMap logo

A Mercator projection for Nearmap imagery

Grounded Patterns near Bendigo
Grounded Patterns near Bendigo

In testing Nearmap’s not-for-profits’ plan, Russell Rigby of NSW reported that after analysis;

  1. The Google Mercator units aren’t even close to metres so current Nearmap imagery cannot be imported straight into OCAD at a meaningful scale.
  2. The OCAD transformation [via registration of image corners to OCAD map] from Google Mercator to Map Grid of Australia is good, but needs testing in more areas to be sure.

Following further investigation by Russell and myself we determined that the EPSG code 3785 referenced by Nearmap was not present in OCAD’s co-ordinate system range. Further detective work showed that OCAD contains the original version of the Google Mercator projection (EPSG 900913) but it is no longer officially in use. Not that a cursory search of the spatial world would tell you that. Fortunately Russell came across this post in Alastair Aitcheson’s spatial blog.

Popular Visualisation CRS / Mercator

I advised OCAD hq of the issue along with the background we discovered and they got onto the matter immediately. Their recent service release 11.1.5.1351 of 2013-09-04 contains the co-ordinate system referenced by Nearmap images available under the latter’s Basic plan. In the OCAD co-ordinate system list it is named Popular Visualisation CRS / Mercator (EPSG 3785). Note this addition is not documented in the release notes.

OCAD EPSG 3785

Images previously downloaded via Hypertiles

If you downloaded images via Hypertiles before Nearmap closed off the free service, then you should find those images are georeferenced. Thus if used as a background image, the images will register nicely with a georeferenced map.

You can check the co-ordinate system of such images by opening the .vrt file that is in each Hypertiles data set. The first few lines show;

[quote] <VRTDataset rasterXSize=”75677″ rasterYSize=”113090″>
<SRS>EPSG:28355</SRS>[/quote]

The EPSG reference can then be looked up at spatialreference.org. In this case [quote]EPSG:28355: GDA94 / MGA zone 55[/quote]

However, whichever you selected when running Hypertiles, OCAD should convert nicely.

Russell Rigby on trialling Nearmap’s Basic plan

(Nearmap’s Basic plan offered to orienteering appears to be the same as, or very similar to, their Photomaps Standard Plan).

There is no equivalent of Hypertiles now, where you can nominate the area and resolution level that you want ( at least at the subscription level that is in any way likely).

The current subscription model has a download limit (50MB/month basic, next level 250MB/month). Any image displayed on the screen is included in downloads, to the extent that Nearmap suggests reducing the size of the browser window as a way of reducing the downloads. Panning or zooming adds to the download, as does viewing an image sequence. Using the screen tools (measuring, area, lat/long etc) does not add to downloads as long as the image is not moved.

There are two choices when saving an image. The first is to effectively capture the screen, the second is to define a box to download at high resolution. Both options can generate a world file, in the format attached to the image I sent previously. The only indication of the size & resolution of the resulting image file comes after the download has been requested and performed, so working on a tight download budget could be difficult.

The jgw is the only world-file format – there is no equivalent of the vrt file. Nearmap help suggests loading the output files into a GIS system (ESRI, Mapinfo etc) to work further with the image.

In the test data area Level 17 corresponds to approx 1m pixel size (1.19 “units”), and each change in level corresponds to a factor of 2 eg level 18 == 0.5m pixel. The only way I can see to control download size is to measure the max dimension of the area I want, & do a quick calculation of which scaling level will give between 3k & 6k pixels along that dimension, or I get to level 21 (approx 6cm) which is the smallest pixel size available.

The quality of the imagery is very high, and for capital city areas, very frequent. Regional coverage is less frequent (3-6 months for Newcastle), & is patchy and one-off for other coastal & inland towns in NSW. Many towns are not covered, or are only photographed in flood time! Some areas I have looked at have more recent coverage on Google or Bing, but usually not as high resolution.

The terms & conditions for the licence say that the first 2 months have unlimited downloads.

 

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VIC, NSW, NZ Release Free Spatial Data

data.vic home page extract

Recent releases

July saw Victoria follow NSW in making available online, free spatial data free. New Zealand’s LINZ released a significant amount of aerial imagery with more to follow. I checked out the VIC release and created a quick guide for those not familiar with accessing spatial data.

Apart from being useful to create base maps, spatial data makes it easy to georeference existing maps. The most useful data sets for orienteering and rogaining are elevation (contours, spot heights), transport (road, rail, tracks), hydro (lakes, streams, watercourses), property boundaries and vegetation density. Most of those data sets have associated infrastructure but some might be obtained from a features data set.

The remainder of this post deals with my quick survey of the VIC offering. The situation is likely to be broadly similar in other states.

Victoria

900 data sets released on data.vic.gov.au

While Orienteering Victoria has had a free data licence for some years, acquiring a data set cost $200 for the work involved. Not to mention the time of orienteers in preparing and submitting the application.

Now you can download seemingly any or all of the six data sets useful to map sports to some degree.

Rail data on map
Preview of rail data overlaid on map

[unordered_list style=”green-dot”]

  • Elevation
  • hydro
  • transport
  • vegetation
  • features
  • property.

[/unordered_list]

Another 100 data sets will be available by September.

Locating your data sets

Easiest is to download the Guide below and search using the ID attached to each recommended data set. If you wish to peruse at leisure then read on.

Each data set may appear in multiple categories. For sport purposes, it appears easiest to search for or select a data set name that includes the words VicMap 1:25,000. These data sets include subsets that otherwise you would need to locate and order individually. For example, Vicmap Hydro 1:25,000 includes subsets watercourses (rivers, streams, channels…), water points (ponds, dams…), water areas (lakes, reservoirs…).

However, the listings are not perfect. For example at time of writing, the Spatial | Transport section does not include VicMap Transport 1:25,000 but does include VicMap Vegetation 1:25,000 (I have notified this).

A quick guide

For the uninitiated, selection of an appropriate data set may seem daunting when you first hit data.vic.gov.au. For example, if you select Transport from the category list shown you end up with social data sets, not spatial data sets. You first have to select Spatial, then the relevant sub category.

And which data format you select depends upon your edition of OCAD. For Professional, select ESRI Shape. For Standard, select Autocad DXF.

To assist, while watching the exciting Tour de France Mont Ventoux stage, I delved into the web site and prepared a quick guide which you can download. It will not be perfect but it should help you avoid long detours. I will update it as necessary (let me know of any errors) and am working on some short video guides.

[ilink url=”https://ocad.com.au/wp-content/uploads/2013/07/Kens-guide-to-free-Vic-spatial-data.pdf” style=”download”]Guide to access free VIC spatial data.[/ilink]    1 page pdf. Updated 17 July 2013.

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Bezier Curves, Illustrator Style

Adobe Illustrator logo

Adobe Illustrator mode for bézier curves

If you are or have been an Adobe Illustrator user, you might prefer to draw bézier curves in OCAD using that method.Bezier curve image

To activate, go to Options | Ocad Preferences | Drawing and Editing | Drawing Bézier Curves | Adobe Illustrator mode and turn it on.

If you have been used to Illustrator on Mac, then be aware that various command keys are different in Windows e.g. Command key in Mac becomes Control key in WIN.

btw

Tradie Tough Paper Notebooks, 160pgs. A7 $2, A5 $3, A4 $3.30. Good for mapping field work notes though I would have preferred a soft cover. @ Officeworks.

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Why Georeference an Orienteering Map?

georeferencing image

What is georeferencing?

In simple terms for orienteering, georeferencing is aligning a map with the national map grid. On a georeferenced orienteering map, any point can be related to its location on the national grid and thus on the ground. The national projection is Map Grid of  Australia 94 (MGA94). The related datum is the Geodetic Datum of Australia 94 (GDA94) and is expressed in latitude and longitude.  GDA94 is consistent within 1 metre to the World Geodetic System 1984 which users of a GPS device may recognise as WGS84.GPS image

Why georeference O maps?

Having just mentioned GPS, the first response is that a georeferenced map enables import of GPS data or a GPX file with automatic correct placement on the map. On a non-georeferenced map you would be faced with moving via mouse, the GPS track and waypoints to the correct position – often a very tricky task. Data from a TruPulse laser rangefinder  can also be captured in the field (Professional edition) and appear instantly in the correct location on the map.

ortho photoA second response is that with a georeferenced map, ortho aerial photos such as provided by NearMap are automatically aligned with your map. Similarly for georeferenced data and georeferenced PDFs.  A huge time saving and likely a far more accurate result when transcribing detail from the photo.

A correctly georeferenced map minimises distortions in the map which can trouble elite orienteers. One map I have run on a number of times troubled even me. I had the opportunity to georeference it recently and found a section of it was so badly distorted that it also affected placement of features beyond that area.

When and how to georeferenceShape & DXF image

Ideally when the map is first created. Sources of data to create a georeferenced base map include Shape (Professional edition) and DXF files from state mapping and local government. Shape files generally offer the most useable data and are easiest to use. Open Street Map data may be useful for some types of map (Professional edition but see free service below).

Georeferencing existing maps is reasonably straightforward if you have access to map data that is already georeferenced. My preference is to georeference against GIS data. This is now free and easy to download in some states including Victoria. I usually use the roads data set from the Transport shape file as road junctions, as opposed to tracks, are fairly accurately positioned.

A scan of a topo map is also reasonable straightforward although a georeferenced digital data map is even easier.

Be prepared for distortions to show up especially if the map was not created by a professional mapper. OCAD will fairly successfully correct slight distortions. But if they are considerable and/or a section of the map is grossly distorted, then the Rubber Sheeting function in OCAD Standard and Professional is likely the only answer. OCAD website has videos on georeferencing and rubber sheeting.

Georeferencing service

While georeferencing is within the capabilities of most orienteering mappers, updating a library of maps may not be best use of your time. I offer a paid service to do the drudgery for you. See the Cartographic Services page for more information.

OSM import free serviceOpen street Map logo

Want to use OSM data but don’t have OCAD Professional edition? Read more...