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I am attempting to clip DEM rasters (USGS, in WGS 84) into a set of tiles measuring 2.56km on a side, in QGIS, using python.

So far, it appears that gdal_translate is working for me, using a bounding box in latitude/longitude coordinates. My problem is that I need to know how to convert 2.56 km into the equivalent distance in latitude and longitude. Thus far I've been getting by in other applications using the following formula, found online here: http://fmepedia.safe.com/articles/How_To/Calculating-accurate-length-in-meters-for-lat-long-coordinate-systems

rLat = math.radians(mapCenterLat)

metersPerDegreeLat = 111132.92 - 559.82 * math.cos(2 * rLat) + 1.175 * math.cos(4 * rLat)

metersPerDegreeLong = 111412.84 * math.cos(rLat) - 93.5 * math.cos(3 * rLat)

degreesPerMeterLong = 1.0 / metersPerDegreeLong

degreesPerMeterLat = 1.0 / metersPerDegreeLat

It's black magic, but it seems like it's worked for me so far. However, in this instance when I plug in that equation, and multiply my resulting degreesPerMeter numbers times 2560, the resulting rectangle ends up being more like 3.45 km wide by 2.49 km high.

Clearly there must be a better way in QGIS to figure this out, without resorting to black magic, right?

Also, at the risk of introducing an unrelated question: Can anyone tell me whether 1/3 Arc Second DEM files, which are listed as 10m height resolution, can be relied upon to have pixels exactly ten meters apart, in both directions? Or is that an approximation?

Many thanks!

qgis dem convert

share

asked 5 mins ago

chriscalefchriscalef

82

New contributor

chriscalef is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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add a comment | 

0

I am attempting to clip DEM rasters (USGS, in WGS 84) into a set of tiles measuring 2.56km on a side, in QGIS, using python.

So far, it appears that gdal_translate is working for me, using a bounding box in latitude/longitude coordinates. My problem is that I need to know how to convert 2.56 km into the equivalent distance in latitude and longitude. Thus far I've been getting by in other applications using the following formula, found online here: http://fmepedia.safe.com/articles/How_To/Calculating-accurate-length-in-meters-for-lat-long-coordinate-systems

rLat = math.radians(mapCenterLat)

metersPerDegreeLat = 111132.92 - 559.82 * math.cos(2 * rLat) + 1.175 * math.cos(4 * rLat)

metersPerDegreeLong = 111412.84 * math.cos(rLat) - 93.5 * math.cos(3 * rLat)

degreesPerMeterLong = 1.0 / metersPerDegreeLong

degreesPerMeterLat = 1.0 / metersPerDegreeLat

It's black magic, but it seems like it's worked for me so far. However, in this instance when I plug in that equation, and multiply my resulting degreesPerMeter numbers times 2560, the resulting rectangle ends up being more like 3.45 km wide by 2.49 km high.

Clearly there must be a better way in QGIS to figure this out, without resorting to black magic, right?

Also, at the risk of introducing an unrelated question: Can anyone tell me whether 1/3 Arc Second DEM files, which are listed as 10m height resolution, can be relied upon to have pixels exactly ten meters apart, in both directions? Or is that an approximation?

Many thanks!

qgis dem convert

share

asked 5 mins ago

chriscalefchriscalef

82

New contributor

chriscalef is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

I am attempting to clip DEM rasters (USGS, in WGS 84) into a set of tiles measuring 2.56km on a side, in QGIS, using python.

So far, it appears that gdal_translate is working for me, using a bounding box in latitude/longitude coordinates. My problem is that I need to know how to convert 2.56 km into the equivalent distance in latitude and longitude. Thus far I've been getting by in other applications using the following formula, found online here: http://fmepedia.safe.com/articles/How_To/Calculating-accurate-length-in-meters-for-lat-long-coordinate-systems

rLat = math.radians(mapCenterLat)

metersPerDegreeLat = 111132.92 - 559.82 * math.cos(2 * rLat) + 1.175 * math.cos(4 * rLat)

metersPerDegreeLong = 111412.84 * math.cos(rLat) - 93.5 * math.cos(3 * rLat)

degreesPerMeterLong = 1.0 / metersPerDegreeLong

degreesPerMeterLat = 1.0 / metersPerDegreeLat

It's black magic, but it seems like it's worked for me so far. However, in this instance when I plug in that equation, and multiply my resulting degreesPerMeter numbers times 2560, the resulting rectangle ends up being more like 3.45 km wide by 2.49 km high.

Clearly there must be a better way in QGIS to figure this out, without resorting to black magic, right?

Also, at the risk of introducing an unrelated question: Can anyone tell me whether 1/3 Arc Second DEM files, which are listed as 10m height resolution, can be relied upon to have pixels exactly ten meters apart, in both directions? Or is that an approximation?

Many thanks!

qgis dem convert

share

asked 5 mins ago

chriscalefchriscalef

82

New contributor

chriscalef is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

rLat = math.radians(mapCenterLat)

metersPerDegreeLat = 111132.92 - 559.82 * math.cos(2 * rLat) + 1.175 * math.cos(4 * rLat)

metersPerDegreeLong = 111412.84 * math.cos(rLat) - 93.5 * math.cos(3 * rLat)

degreesPerMeterLong = 1.0 / metersPerDegreeLong

degreesPerMeterLat = 1.0 / metersPerDegreeLat

share

asked 5 mins ago

chriscalefchriscalef

82

New contributor

chriscalef is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share

asked 5 mins ago

chriscalefchriscalef

82

New contributor

chriscalef is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 5 mins ago

chriscalefchriscalef

82

New contributor

chriscalef is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 5 mins ago

chriscalefchriscalef

82