# Heightmaps

## What are heightmaps?

In principle, heightmaps are just images. However, they are special in two ways:

* the images are usually Geo TIFFs, so that every pixel is georeferenced, i.e. can be mapped to a precise location on earth
* the images are usually grayscale, because the value of each pixel signifies the height of that location's ground above a height reference (like e.g. the WGS84 ellipsoid)

**SpatialExplorer's** **MissionGuidance** can use these heightmaps for two different purposes:

* For an accurate ground elevation visualization in **AGL Oracle**
* To recompute flightlines' altitudes before using them in **MissionGuidance**

## How to retrieve heightmaps

In order to download the right heightmaps for your project, go to <https://grids.phoenixlidar.com>.

Use the "Filter by grid type" button to select either the **SRTM 3 arcsecond** or **ASTER** datasets.  Use **ASTER** height maps when you plan to acquire data above **60 degree N** latitude or below **56 S**. For operations at **mid-latitudes**, users can use **SRTM** data sets.&#x20;

Once you have filtered by data set type, navigate to the project area. Select all tiles that are covered by your project and download them:

![Heightmaps available for download at https://grids.phoenixlidar.com](/files/-MkLjruto4XLgPC9d5uf)

The files need to be placed in your **\Documents\Phoenix LiDAR Systems\HeightMaps** folder (this folder is created automatically on SpatialExplorer's first startup). If you have downloaded zipped versions of the heightmaps, please extract the **TIF** files manually into the same folder.

## Recomputing Flightplan Altitudes Using Heightmaps

![Using downloaded heightmaps to recompute flightline's altitude above ground (before)](/files/-MkLlBXhQJVmYVD6DGXz)

Above is an example of a flight plan created using 3rd-party software. Sometimes these flightlines will not follow the terrain accurately, or may contain large steps/discontinuities in height. In other cases, the 3rdparty software may not have exported height at all, or using an incorrect datum. In these cases, use the functionality shown above to "reflow" the flightplan above the terrain before flying the project. Be aware that reflowing of flightlines **does not add waypoints** to flightlines even when extreme changes in terrain height may require it.

![Using downloaded heightmaps to recompute flightline's altitude above ground (after)](/files/-MkLlRccp6EzvdkP4CwK)


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