CHAPTER 11

Creating a Viewshed

EcorisMap can calculate the area visible from a given point (the viewshed) from elevation data and draw it on the map. It is useful for checking the line of sight from a raptor observation point, or for choosing survey and observation locations. The viewshed is a temporary display — it is not saved as a layer or data.

How to create one

1. Long-press the spot on the map you want to use as the observer point.
2. Tap “Create Viewshed” in the popup.
3. Enter the distance and the observer height in the “Create Viewshed” dialog, then tap “OK”.
4. When the calculation finishes, the viewshed appears on the map and a banner with the count is shown at the top of the screen.

The calculation runs on your device, so a larger distance takes longer (a 10 km radius loads about 120 elevation tiles). A loading indicator is shown while it runs — wait until it finishes.

Dialog items

• Distance (km) — The radius from the observer point. Enter a value between 0.1 and 100 km (default 3 km). Beyond 10 km, the resolution of the elevation data becomes coarser with distance.
• Observer height (m) — The height of the viewpoint above the ground. Enter a value between 0 and 1000 m (default 2 m). Visibility is calculated from the observer’s ground elevation plus this height.
• Use point ”…” as center — Shown only when a visible point feature is near the spot you long-pressed. Turn it on to use that point’s coordinates as the observer instead of the long-pressed position. Use it when you want to calculate from an existing point such as a fixed observation point or a nest.

How it is displayed

• The visible area is filled in semi-transparent red, together with a circle of the distance you specified and a numbered observer marker. If the visible area is split into separate patches, each is drawn on its own, and areas that are not visible are represented as holes.
• Long-press and create again while a viewshed is displayed to overlay the viewsheds of several observer points. The numbers follow the order of creation — handy for comparing the line of sight from different places or with different settings.
• While viewsheds are displayed, a banner with the count is shown at the top of the screen. Tap the trash button on the banner to clear all displayed viewsheds after a confirmation.
• The viewshed is a temporary display. It stays visible through other operations such as switching maps, but it disappears when you quit the app. To keep the result, take a screenshot.

About the elevation data

• Inside Japan the elevation tiles of the Geospatial Information Authority of Japan (DEM10B, about a 10 m mesh) are used; where the area extends outside Japan, it automatically falls back tile by tile to AWS Terrain Tiles (global coverage).
• An internet connection is required to fetch the elevation tiles. Tiles that have been loaded are cached on the device, so recalculating with a different distance or height is faster. If some tiles cannot be fetched because of a network error, creation is cancelled so that an inaccurate result is not displayed.
• To create viewsheds where there is no signal, download “Elevation tiles (for viewshed)” in download mode beforehand (mobile only; see Download Maps for Offline Use). Within the downloaded area you can create viewsheds offline.
• The calculation corrects for the curvature of the earth and for atmospheric refraction.
• The elevation data is the height of the ground surface. The height of trees and buildings is not taken into account, so treat the result as a guide.
• Where no elevation is available, such as at sea, “Failed to get elevation data” is shown and the viewshed cannot be created.

Note: The popup shown by long-pressing the map also displays the elevation of that spot (see The Home Screen).