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Last Post 01 Mar 2017 09:55 AM by  Paulo Olivas
The vectors appear to be shifted one half pixel
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Paulo Olivas



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01 Mar 2017 09:55 AM
    Hi there, I am trying to generate DSM (1m) and DEM (2m) from LIDAR data, but the problem I am experiencing is that the resulting DSM and DEM have one half pixel shifted. I tried to change extent in multiple ways but I was not successful. Any suggestions. Paulo

    Mari Minari



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    01 Mar 2017 10:04 AM
    Is it the DEM that is shifted or the vector files created for the products? I found a report that the output vectors may be shifted due to an issue in the projection engine. This issue was fixed in ENVI lidar 5.2 SP1. What version of ENVI Lidar are you using?

    Paulo Olivas



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    01 Mar 2017 10:24 AM
    Hey there, I am using 5.4. I see the shift in the DEM.

    Paulo Olivas



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    01 Mar 2017 10:28 AM
    So would you recommend to just shift the image? 

    Mari Minari



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    01 Mar 2017 10:54 AM
    I haven't heard of any similar reports. How are you determining that there is a shift in the DEM? Are you looking at the extents of the the output DEM compared to the point extents? When processing Lidar points, there is a small border that will be excluded in the products.

    Paulo Olivas



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    01 Mar 2017 11:20 AM
    when I compare the area selected in the "area definition" tab with the extent of the DEM or DSM the pixels are shifted 0.5. For instance, I want a DEM/DSM that are on a even grid. So when I select the area to start in a even pixel the resulting DEM/DSM is shifted 0.5.

    Mari Minari



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    01 Mar 2017 11:45 AM
    When you set the area definition, you are specifying the pixels at those locations which are points. The actual values of the corners of the pixels will be different depending on the output pixel size chosen for the DEM/DSM. So in this case, if I set my extents to be: xmin: 593800 xmax: 594000 Ymin: 5289600 ymax: 5290000 My output DEM using 1m pixel size 'extents' come from the upper left corner of the upper left pixel and lower right corner of the lower right pixel in the image: xmin: 593799.5 xmax: 594000.5 ymin: 528599.5 ymax: 5290000.5 If you go to these two corner pixels in the image and select the center of the pixel, it should match the 'extents' specified in the Area Definition.

    Paulo Olivas



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    01 Mar 2017 11:59 AM
    That is my problem. How do I prevent that from happening? I tried adding/subtracting 0.5 to the initial extent to see if this could work, but it did not.

    Paulo Olivas



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    03 Mar 2017 12:22 PM
    So would you recommend to just shift the image? 
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