I'm writing with a question regarding unexpected returns on a slope analysis using DTM LiDAR data at the scale of 2m in QGIS.

This is for my MSc dissertation (UK) which is due in about 6 months, so I want to be thorough.

I carry out the analysis I transformed the DTM raster data into slope raster data using QGIS slope Raster Terrain Analysis function - output format = GeoTIFF with Z factor of 1. I then converted the raster slope data into vector data (polygon) using the raster to polygon function. When I review some points on the map, they don't seem to align with what I know of the land (a few degrees off).

I should note that I have my map (including vector point data) in the WGS 84 Pseudo Mercator CRS whereas my DTM in data requires OSGB 1936 to overlap.

Perhaps I'm missing something, or are the slope outputs are correct?

LiDAR data was sourced from link provided below. https://data.gov.uk/harvest/gemini-object/33e78ae5-27d9-44db-9e2c-2e75c049e09c

closed as unclear what you're asking by Andre Silva, nmtoken, whyzar, mgri, Mark Ireland Feb 14 '17 at 15:37

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • What is the source of "the land" knowledge? – FelixIP Feb 13 '17 at 20:14
  • When asking a question like this, take care to add a tag describing the software / library you are using – atlefren Feb 13 '17 at 20:16
  • The land is of Edinburgh city - I live in the nearby city of Glasgow and have visited the city many times. I recently visited the very specific points where I need to slope data for inspection – bikingman Feb 13 '17 at 21:17
  • This question is unclear. What are exactly the unexpected results? Would you mind showing them to us (screenshots?). What happens when you compare the DTM raster to "what you known of the land"? Do they align? Why not using the same CRS for vector and raster data? Are you projecting them correctly? Also, what do you mean with "map"? Is it the converted raster slope to polygon? Please, kindly clarify. – Andre Silva Feb 14 '17 at 11:51

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