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bdg49 post(s)
#27-Sep-19 19:17

I downloaded and unzipped a US land cover image yesterday. https://s3-us-west-2.amazonaws.com/mrlc/NLCD_2016_Land_Cover_L48_20190424.zip. After linking in M9 I overlaid some tigerlines to orient myself in my own neighborhood. To my surprise there was a displacement of about 4350 meters between the features of tigerlines and the land cover image. The image being north of where the roads and river feature of tiger lines would suggest they be. The same datasets in QGIS resgister properly.

tjhb

8,883 post(s)
online
#27-Sep-19 20:52

Can you give more details?

What version and build of Manifold.

What steps did you follow to link the image and (especially important) to check or adjust the projection? (What did you click? That much detail.)

Same for the drawing(s).

Then you combined them in a map.

bdg49 post(s)
#27-Sep-19 23:16

Manifold 9.0.169.8

New project.

File link the img file (NLCD_2016_Land_Cover_L48_20190424.img) from the unzipped set.

Right click on tiles table, click schema, dbl click the compass, see Lambert Conformal Conic. Click another compass and see C.lat 23, c.Lng -96, 1st 29.5, 2nd 45.5.

File -> Import <some Dir>tl_2019_25017_edges.shp -- whereever you downloaded it.

Right click on the the shp's table, dbl click on the geom's compass, "GCS_North_American_1983"

Create map, default EPSG:3857.

Open map.

Drag in NLCD image.

Drag in Tigerlines (tl_2019_25017_edges)

Zoom into -71.286977 42.617 1:450000

The blue lines in the image below suggests where river from TL and river from image differ.

Attachments:
offset.PNG

bdg49 post(s)
#28-Sep-19 11:12

A bing map background image is faster and easier to to demonstrate the issue than is downloading and unzipping a shpfile.

oeaulong

218 post(s)
#28-Sep-19 18:04

I am importing it to Mfd9 taking aprox 30 min on my machine. Haven't tested yet, but I have a question.

Right click on tiles table, click schema, dbl click the compass, see Lambert Conformal Conic. Click another compass and see C.lat 23, c.Lng -96, 1st 29.5, 2nd 45.5.

Why do you believe that it is Lambert? From the "NLCD_2016_Land_Cover_L48.xml" file, the spatial projection is reported to be Albers with a datum of WGS_1984. The parameters are as you posted.

<mapproj>

<mapprojn>Albers Conical Equal Area</mapprojn>

<albers>

<stdparll>29.5</stdparll>

<stdparll>45.5</stdparll>

<longcm>-96.0</longcm>

<latprjo>23.0</latprjo>

<feast>0.0</feast>

<fnorth>0.0</fnorth>

</albers>

</mapproj>

Could fixing the initial projection be simple solution? If you are reading this before I finish testing this hypothesis try it and see.

oeaulong

218 post(s)
#28-Sep-19 18:41

Confirmed. I imported, overlaid Bing streets and saw your 4k displacement. Checked the default projection for NLCD and yes it came in with Lambert. I "Repaired Initial Projection" and set it to Albers Conical Equal Area and then changed the Base (datum) to WGS84. on opening the map, it lined right up. I don't know why it defaulted to Lambert, but as a matter of course, I always double check with the metadata and never trust the imported values. This is a fine example of why.

bdg49 post(s)
#28-Sep-19 19:42

Thanks for the effort and and report. Checking it now.

bdg49 post(s)
#28-Sep-19 20:11

Thank you for the lesson. Proper alignment confirmed.

tjhb

8,883 post(s)
online
#29-Sep-19 06:17

Nice work.

... as a matter of course, I always double check with the metadata and never trust the imported values.

Or vice versa (I think you would agree). It's great to have two things to check.

Very often, at least one is right.

adamw


8,696 post(s)
#02-Oct-19 10:37

By the way, we found the reason the original file was being read as Lambert Conformal Conic - this was a bug in our code. We will fix the bug and the file will read as Albers Conical Equal Area on first try without any further tinkering required.

bdg49 post(s)
#02-Oct-19 11:24

thanks for the update

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