Point value Change after projection - USGS DEM Raster VS Imagery Layer from Esri

Question

Hello Tech.

I'm working on a project that requires National level DEM data, and I'd like to thank ESRI for providing the imagery layer (https://www.arcgis.com/home/item.html?id=0383ba18906149e3bd2a0975a0afdb8e).

* For the most part, the extracted point values from the USGS raster and the ESRI imagery layer are very close. (Col 2 , Col 3 &Col 6)
* However, when I project those raster’s (WGS 1984 Web Mercator Auxiliary Sphere), the point value difference between projected raster’s from USGS and ESRI were significantly high. (Col 4, Col 5 & Col 7)

Shape *

Raster exported from USGS

Raster exported from ESRI_layer

USGS_Projected
          (WGS-1984)

ESRI_layer_Projected
             (WGS-1984)

Abs(Raster exported        from USGS-Raster    exported from   ESRI_layer)

Abs(USGS_Projected (WGS-1984)-ESRI_layer_Projected (WGS-1984))

Point

3071.917

3071.913

3032.042

3071.913

0.004

39.871

Point

3268.988

3268.984

3297.935

3268.984

0.004

28.951

Point

3496.321

3496.318

3488.198

3516.118

0.003

27.92

Point

2501.627

2527.583

2501.627

0

25.956

Point

3253.484

3253.491

3246.843

3271.352

0.007

24.509

Point

2932.126

2932.129

2932.126

2955.189

0.003

23.063

Point

3088.094

3088.087

3088.094

3110.907

0.007

22.813

Point

2760.626

2760.628

2789.526

2766.848

0.002

22.678

Point

2647.81

2647.816

2625.496

2647.816

0.006

22.32

Point

2546.791

2546.793

2568.551

2546.793

0.002

21.758

Point

2973.761

2973.768

2955.436

2976.748

0.007

21.312

Point

2899.172

2899.171

2910.918

2889.789

0.001

21.129

Point

1535.169

1535.16

1535.169

1514.62

0.009

20.549

I tried various geographical transformations and resampling methods but received no improvement. What could be the cause of this, and how should I proceed?

Looking forward to hearing from You

BojanŠavrič · Accepted Answer

Hi @Jaslam,

I might have an answer for you, but I need a bit more information before I write a proper reply.

In which Projected/Geographic and Vertical Coordinate Systems is your Raster exported from USGS and Raster exported from ESRI_layer data?

I assume your target VCS for both datasets is WGS 1984, WKID::115700. Is this correct?

Thanks,
Bojan

BojanŠavrič · Accepted Answer

@Jaslam,

We look at your workflow and I might be able to provide you with an explanation.

These differences are caused by a resampling technique to source the projected rasters. In your case, this would be the Nearest Neighbor method. Since your USGS_1_n45w114_20130911 and ESRI_shape have different extents and raster grids, when they are projected to WGS84 Web Mercator, their projected grids aligned differently with the source raster grids and the Nearest Neighbor method picks different pixel to source its raster value. E.g. take your point with 3071.913 height from ESRI_shape_ProjectRaster layer and check all nearest 9 pixels round that point in USGS_1_n45w114_20130911. One of the pixels will have the height of 3032.042 as it is in USGS_Proj_NAD_1983_To_WGS_1984_5 layer. Differences vary based on the elevation changes in the point’s neighborhood. Some are higher, some lower, and some are exactly equal. Similar effect, but with a bit smaller differences happens with other resampling techniques for the same reason.

Please note that the Nearest Neighbor method is inappropriate for elevation surfaces. For more details, please check Project Raster online documentation page.

I hope this helps,
Bojan