FAQ: Why is the data I converted to NAD27 partially missing or of low accuracy?

Conversions to the NAD27 datum are calculated with the Geosoft transform:

• [NAD27] Canada NTv2 (12 min)

This transform is based on the highly accurate Geomatics Canada's National Transformation Software Package: NAD27 - NAD83 (NTv2). However, this conversion is only supported over the Canadian landmass and in the adjacent near-shore. Data located outside this area are deleted from the output dataset. Areas outside the coverage of NTv2 should be converted using the Geosoft transform:

• [NAD27] MEAN Canada

This transform is based on formulae presented in NIMA TR8350.2, National Imagery and Mapping Agency, US DOD. While this transform provides worldwide coverage, it is of lower accuracy than NTv2.

Q : How do I download data directly using Geosoft's Oasis montaj DAP interface (free or full version)?

A : Read a description of Geosoft's Oasis montaj viewer and download a tutorial that describes how to download data from a DAP server.

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Q : What is your DAP server's URL (for use in Oasis montaj)?

A : http://gdrdap.agg.nrcan.gc.ca (Oasis montaj v.6.4)

Q : How do I import the "Arcview Binary Raster" format (.FLT file) into ArcView (or ArcMap/ArcGIS)?

A : It depends which version of ArcView you use (other versions may be similar) :

• ArcView 3.2 : requires the Spatial Analyst component
• ArcView 8.2 or 8.3 : import the .FLT file with the converter found in the following location :
  • in ArcToolbox,
  • choose Conversion Tools
  • choose Import to Raster
  • choose Floating Point Data to Grid
• ArcView 9.0 or 9.1 : import the .FLT file with the converter found in the following location :
  • in ArcToolbox,
  • choose Conversion Tools
  • choose To Raster
  • choose Float to Raster

  This converts the .FLT to an ESRI Grid format.

Q : I get an error: "ArcToolbox warning: Required header file does not exist."

A : Try shortening the filenames, and the number of subdirectories in the path (ie. move the files closer to your root directory).

Q : I don't have software to process gridded data. How can I make image files from these data?

A : Images of gridded data can be rendered using software available as a free download from GEOSOFT Inc. The Oasis montaj Viewer can be used to display gridded data as colour shaded relief maps. These maps can be exported in a variety of standard bitmap image formats (.tif, .jpg, .gif etc.). In addition, the software has a taskbar button that allows one to define a map area, download data from the GDR DAP server, and automatically display the map image.

The following instructions are for version 6.3.1 of the free Oasis montaj Viewer.

Download the free Oasis montaj software (v6.3.1)

To create images:

1. Download and install the free Oasis montaj Viewer.
2. On the Oasis montaj task bar, click on 'File', then 'Project', then 'New'.
3. Enter a New Project Name.
4. On the task bar, click on the 'DAP' button, then 'New Map'.
5. Follow the dialogue boxes to define a map area and create a new map.
6. Click on 'DAP' again and then 'Find data'. Click on 'Servers' and enter the URL http://gdrdap.agg.nrcan.gc.ca
7. Authorize Internet Communication for the GDRIS DAP server
8. In the 'Get DAP Data' dialogue box, click on the 'Canada: Geoscience Data Repository' Dataset
9. Click on the desired data sub-type (e.g. Aeromagnetic, Gravity, Lithoprobe or Radioactivity)
10. Click on the desired data form (Compilation/Survey, grid, etc)
11. Click on the desired data layer to select it
12. Click 'Get Data'
13. Set Options in the 'Data Format Options' area as required.
14. Click 'Get Data'

The software will display the data as specified under step (13) above.

To export images:

1. Click on 'Map' on the taskbar.
2. Click on 'Export'
3. Choose an 'Output Format' from the dropdown menu.
4. Set the Image Resolution to your requirements.
5. Click 'Ok'.
6. Enter a name for the image file in the 'Exported file name' dialogue box.

Q : While extracting data, specifying the datum and projection information changes the data layer name and I end up downloading the wrong data set. How can ensure I get the right data?

A : DAP resets the data layer name to the active data layer on the display window after datum and projection information are provided. Simply reselect the data layer you wish to download and click on 'Ok'.

Q : Where do I enter latitudes and longitudes to zoom to my area of interest?

A : Depending on the resolution and size of your monitor, the lat/long extents box may not be visible. Scroll down to reveal the box.

Q : I specified a projection for a grid that was different from the default projection. I also set the resolution of the grid. However, the downloaded grid had a different resolution than I requested. How can I get a grid with the requested projection and resolution?

A : The default projection and resolution of grids are those of the grid as stored in the application. If you choose a different projection, the application will calculate a different resolution. You may reset this calculated resolution, but the final output grid resolution will be 10% to 20% larger than requested. This is a concession to processing speed made by the software.

If you download gridded data from the GDR server via Geosoft's Oasis montaj Viewer software, you will receive the grid with the projection and resolution you request.

Q : Is there a help tool for the DAP application?

A : The help tool for the DAP application can be accessed on:

• the Help button on the top menu bar of the application
• the icon on the application page