How to calibrate DOQ and DOQQ files

Questions and problems regarding PathAway Map Manager

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Bill Zwicky

How to calibrate DOQ and DOQQ files

Postby Bill Zwicky » Mon Mar 06, 2006 2:54 am

This has been driving me up a wall, so I thought I'd post this for posterity.

You can get free orthophotos (like satellite photos) for Illinois from the ISGS, specifically the DOQQ files. I believe DOQ files are available for the rest of the US.

USGS has much tastier hi-res color scans of large cities across the US, including Chicago and St. Louis. They're not on the ISGS site yet, but you can get them all from the USGS's own site. I converted Navy Pier with the TouraTech demo, and they look nice. But they're limited to the big cities, while the DOQQ files cover the entire state.

The DOQQs come with "world files", but these don't work in Map Manager. Neither does the MrSID format of the images. And TouraTech failed to calibrate correctly, for some reason. But with these directions, Map Manager will give you perfect calibration, quick and easy.

So you'll need some tools:
* Get the doqmeta.zip mentioned on the DOQ page. It's a DBase file, but OpenOffice Spreadsheet can read it.
* So you might need Open Office too.
* Get the MrSID decoder from LizardTech.

Follow the instructions included with the decoder, and convert your *.sid files to *.bmp.

Start Map Manager, and import the bmp file. This will take a while.

Open the doqindex.dbf file. Look for the base name of the sid file you converted in the first column. i.e. If you converted 36089H21.sid to 36089H21.bmp, then look up 36089H21 in the first column. Scroll across to the columns named W_LONG, E_LONG, N_LAT, and S_LAT. Note that the longitude values are backwards: W_LONG contains the eastern longitude, and E_LONG the western.

Switch Map Manager to calibrate mode.

Here's the first critical part: Map manager has two ways to specify Known Points, and one of them is wrong. You must select "Known Points" under Calibration Type, then select "Known Points" again under Coordinate Type. If you use "Lat/Long (Rectangular)", the calibration will be off by around 100 ft.

Also select Map Datum "North_American_Datum_1983" (simply called NAD83 everywhere else) and Coordinate Grid "DD.dddd".

Here's the second critical part: Find the tick marks! The coords in doqindex.dbf do NOT refer to the corners of the image. A short distance in from the corner of each map is a small white cross. If you View->Actual Size then pan to each corner, the cross should already be on your screen. But it might be hard to find if there are any buildings under it. Note also that there are two crosshairs at each point; one is solid white and the other is dashed. For some reason, they're very blurry in these digital images. Try to find the lines that are the brightest white.

Create four known points, centered at each cross. Then mix and match the numbers from doqindex to form the four coordinate pairs, and enter them into Map Manager.

Click Apply, then save the file.

You're done!

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